The letter is still in the news:

http://www.china.org.cn/china/2015-01/30/content_34686142.htm

Emperor Qianlong's letter strategic, not arrogant By Tom CunliffeJanuary 30, 2015 Oxford professor Henrietta Harrison's new research shows that the famous letter from China's Qianlong emperor to Britain's King George III in 1793 was a diplomatic manoeuvre rather than a reflection of arrogance. Harrison's standpoint, presented in a lecture last week, completely overturn's the popular view of Qianlong's actions.
When Lord George Macartney visited China as the first British envoy in 1793, Harrison claimed Emperor Qianlong sensed the military threat that Britain posed to the Qing Dynasty, and far from being arrogant, was actually employing an intelligent diplomatic strategy.
In research on the history of relations between China and the West, there is one letter that can't be ignored. Macartney led the first British mission to China in 1793 and was granted an audience with the Qianlong Emperor in Beijing. The most important requests they made to Qianlong were for trading restrictions to be relaxed, the procurement of a small island near Zhoushan for British traders to reside and store their goods on, and for the establishment of a permanent British Embassy in Beijing. Qianlong presented Macartney with a letter for King George III, refusing these requests outright.
One line from this letter reads: "As your Ambassador can see for himself, we possess all things. I set no value on objects strange or ingenious, and have no use for your country's manufactures." This sums up the contents of the entire letter and was taken by the British, and by many historians since, as evidence that Qianlong was arrogant and ignorant in relation to foreign affairs - thinking that China was at the centre of the world. His comment about "strange and ingenious" objects appears to reject Western technology and the products of the Industrial revolution. This has been used to explain why China missed its opportunity of modernizing and opening up to the world.
Professor Harrison's lecture in Edinburgh on 14th January, involved a thorough deciphering of this letter and her standpoint completely overturns this popular way of looking at Qianlong's actions. Harrison told China.org.cn that she is planning a paper on this subject which she plans to submit for publication later in the year.
So what could Qianlong's real intention have been when he wrote this letter? Harrison states that Qianlong sought an excuse and used this letter to decline the trading requests which were beneficial to the British side, hoping that this would make them leave as quickly as possible. At the same time, Qianlong was also worried that this letter would enrage England, and would cause them to start a war. For this reason he hurriedly deployed coastal defence. Before looking at how Harrison reached these conclusions it would be useful to briefly map out the historical context.
There is a long history of European contact with China. European artisans were already at the Great Khan's court at the time Marco Polo arrived there in the 13th Century. During the 16th and 17th centuries, priests like Matteo Ricci made the journey to China, studied Chinese, and tried to spread Catholicism around China. During the 16th century, the trade between China and Europe also started. Eventually, Western trade was limited to the Southern port of Canton (Guangzhou). This was known as the Canton trade system period (1757-1842). The Chinese government issued trading licenses exclusively to authorised merchants. These merchants would be the only ones allowed to deal with Western traders. This system helped keep trade with foreign countries running smoothly and, by extension, helped to eliminate the possibility of external threats. In Britain's case, as the demand for tea rose and the production of manufactured goods increased during the Industrial Revolution, it wanted to develop and increase trading opportunities and to establish diplomatic relations with China. This led to Macartney's famous British mission to China in 1793.
Overturning History: New Views
Harrison says that the ideas about Qianlong's true intentions became clear from discoveries made available in a book published in 1996 containing all sorts of archival records and edicts written and published by the government and the Qing Ministry of Defense in relation to Macartney's visit to China. She found that when Macartney left Beijing, Qianlong issued many documents outlining the need to strengthen military defence, and on guarding against surprise attacks by Britain. Qianlong issued orders to closely guard the coastal ports. One of the main points Qianlong made was that Britain was demanding that China assign some areas near Zhoushan or Guangzhou for them to set up trading bases to make it easier for them to trade. "We must not only observe the coastlines carefully, but also prepare military defence, especially in Zhoushan and Macao. We must prepare our soldiers in advance to avoid Britain capturing (our land)." This shows that Qianlong was aware of the potential threat Britain posed, and could help explain his actions in rejecting British advances.
The Canton trading system guaranteed Chinese traders a monopoly over foreign trade, whilst protecting the periphery, and Qianlong did not want this to change. Another order which Qianlong issued however specifically stated that his officials should not raise taxes on British ships, especially large merchant ships, as this would only give Britain an excuse to attack.
Why might Qianlong have been so concerned and worried about a possible threat? As Harrison points out, the American Scholar Matthew Mosca's recent book "From Frontier Policy to Foreign Policy: The Question of India and the Transformation of Geopolitics in Qing China," indicates that at around the same time as Macartney arrived into China, the Qing government arrested a Gurkha spy. They found out from him that Britain had taken political control over the province of Bengal. Qianlong then realised that Britain posed a great threat to the areas beyond the Himalayas. This point is key, as Britain first began its colonization of India by establishing a trading presence at certain points along the coast in the 1750s. By the end of that century Britain had attained a huge military dominance there and British rule was being continually consolidated over other areas of India in the ensuing years.
All in all, around 600 similar documents have given Harrison the impression that Qianlong's actions show that rather than being arrogant and ignorant about foreign diplomacy, he considered the arrival of the British envoys to represent a real threat, and made military defence the highest priority. His letter to George III was a means to an end.
Another point which supports this contention is the fact that Qianlong was actually extremely interested in Western technology, and so the traditional view of him having no interest in it is wrong. Throughout the Qing Dynasty, Catholic Jesuits had contact with the imperial family and they had brought some of the "strange or ingenious things" to Qianlong before Macartney's visit. Qianlong collected many superior clocks, including an 18th century English copper plated white crane clock, which was made by a famous clock-smith in London called James Cox. Qianlong also collected telescopes invented by the astronomer William Herschel, who was based in England.
In recent years many scholars have taken onboard new historical standpoints on the history of Sino-British affairs, but old narratives about Qianlong's ignorance persist and are still cited.
Looking back: Reconstructing history
Harrison points out that the traditional views on Qianlong's arrogance actually originate from Chinese scholars at the start of the 20th century. Promulgating this viewpoint was beneficial to their politics at the time. This was then taken up by American scholars and spread in the West. Eventually, Qianlong's letter to King George III was taken as evidence of China's "closed-door-policy."
In 1928, a group of Chinese historians published a book containing 47 documents, out of around 600, relating to the Qing government and its Ministry of Defence. They had some people amongst them who were once members of the Alliance society which was founded by Sun Yat-sen. Following the Kuomintang's anti-Manchu attitude, they selected the documents which were most negative towards the Qing Dynasty and the Manchu people. These selected documents focused on things like border-area defence policies and the famous literary inquisitions, and also included Qianlong's letter to King George III. They did not however choose to publish any of the documents related to military defence against the possible threat Britain posed, already outlined above, which were distributed after Macartney left Beijing. This 1928 book reinforced the mistaken assumption that Qianlong had no idea about Britain's power and was simply being arrogant. Similar instances happened later too with publications in 1941 and 1954 offering similar subjective viewpoints.
Discussing the issue of whether the European perspective traditionally views Macartney's unfair treatment as being one of the reasons that led to the Opium War, Harrison argues that after Macartney's trip, many British sources all played up the "Kowtow rituals," and in the 19th century, the general story was that China was arrogant and not willing to deal and trade equally with Britain. Due to this, and further rejections of equal trading opportunities within China, Britain launched two Opium Wars (1839-1842 and 1856-1860). Britain justified these wars by claiming that its "trading requests" were constantly denied.
Finally, Harrison also stressed the fact that 18th century Europe never had a notion of equal diplomatic relationships with other sovereign states and the hierarchy between different countries was strict. Although the 1647 Peace of Westphalia recognised the concept of sovereign states and that each one is equal under international law, this was not applied to the European powers' imperialist relationship to other countries and continents.
These new findings go some way to show how the writing of history often serves specific interests. In this case, the old narratives revolving around Qianlong's letter favoured Britain and helped to justify imperialism. It will be interesting to see if this new area of research on Qianlong's motives will start to be included in books and textbooks on China.

Click on the link below for a pdf with the presentation. It's over 16 mb (after I squashed it a bit, it was over 25 mb to start).

http://china.usc.edu/sites/default/files/article/attachments/dube-late%20imperial%20china-1000-1800.pdf