1890.] on Astronomical Telescopes. 165 



During the eighteenth century immense advances were made in 

 both kinds of telescopes. With the invention of the achromatic 

 telescope by Hall and Dollond, the long-focus telescopes dis- 

 appeared. 



Newton had turned to the reflecting telescopes believing from his 

 investigations that the dispersion and refraction were constant for all 

 substances ; this was found not to be so, and hence a means was 

 possible to render the coloured fringe that surrounds bright objects 

 when a single lens is used less prominent, by using two kinds of 

 glass for the lens, one giving more refraction with somewhat similar 

 dispersion, so that while the dispersion of one lens is almost corrected 

 or neutralised by the other, there is still a refraction that enables the 

 combination to be used as a lens giving an image almost free from 

 colour. 



In 1733, Hall had made telescopes having double object-glasses 

 on this plan, but never published the fact. Dollond who had worked 

 independently at the subject, came to the conclusion that the thing 

 could be done, and succeeded in doing it ; the invention of the 

 achromatic telescope is with justice, therefore, connected with his 

 name. 



Although this invention was a most important one, full advantage 

 could not be taken of it owing to the difficulty of getting disks of 

 glass large enough to make into the compound object-glass, disks of 

 about four inches being the largest diameter it was possible to 

 obtain. With the reflecting telescope, unhampered as it always has 

 been by any except mechanical difficulties, advance was possible, and 

 astronomers turned to it as the only means of getting larger instru- 

 ments. Many most excellent instruments were made on the Newtonian 

 plan. The plan proposed by Gregory was largely used, as in this 

 instrument objects are seen in their natural position, so that the 

 telescope could be em23loyed for ordinary purposes. 



Many were also made on the plan proposed by Cassegrain. The 

 diagrams on the wall enable you to at once see the essential points 

 of these different forms of reflectors. 



About 1776 Herschel commenced his astronomical work ; beginning 

 with reflecting telescopes of six or seven inches, he ultimately suc- 

 ceeded in making one of four feet aperture. With these instruments, 

 as everyone knows, most brilliant discoveries were effected, and the 

 first real survey of the heavens made. 



Herschel's larger telescopes were mounted by swinging them in a 

 surrounding framed scaffolding that could itself be rotated. The 

 smaller ones were mostly mounted on the plan of the one now before 

 us, which the Council of the Eoyal Astronomical Society have kindly 

 allowed me to bring here. The plan nearly always used by Sir 

 William Herschel was the Newtonian, though for the larger instru- 

 ments he used the plan proposed years before by Le Maire, but 

 better known as the Herschelian, when the observer looks directly at 

 the large mirror, which is slightly tilted, so that his body does not 



