ASTRONOMICAL INSTRUMENTS. 183 



is due to Huyghens, and others about his time, who, still bound by 

 the same necessity of having a long focal length, were^ in conse- 

 quence of the improvements in the manufacture of glass, not bound 

 to the same dimensions. Among the contributions from Holland, 

 and from our own Royal Society, will be seen a large number of 

 lenses, some of them of truly enormous focal length, one of them 

 extending to 360 feet. Although these lenses were made in the 

 infancy, so to speak, of the science, and although they are simple 

 single lenses, the idea of achromatism being one which was intro- 

 duced very much later, the long focal length, combined with their 

 exquisite figure, makes them astronomical instruments of very 

 great power, although, as they were at first used, it was almost 

 impossible to extract valuable observations from them. The 

 instruments of this kind belonging to the Royal Society, were some 

 years ago erected on a special stage in the Kew Gardens (also 

 among the objects exhibited), and Mr. De la Rue found that the 

 figure was absolutely perfect. Since that time, the introduction of 

 the siderostat, perfected by Foucault, although the idea of its 

 introduction was suggested by Hooke, has afforded another oppor- 

 tunity of judging of their performance, resulting in the opinion of 

 Mr. De la Rue being amply indorsed ; and some of them are now 

 being used for the purpose of obtaining enlarged images of the 

 sun, to permit of photographing the disc of the sun, and the spec- 

 trum of the various portions. 



As is well known, it was the opinion of Newton, who lived in 

 the time of the introduction of these long lenses, that the improve- 

 ment of this kind of instrument, in the matter of correcting the 

 coloured effects of dispersion, was " desperate ;" recourse to 

 reflection was therefore suggested, and as a matter of fact the 

 reflecting telescope dates from Newton's time. Newton's original 

 instrument, by which he demonstrated that by employing a mirror 

 instead of a lens a telescope could be made of a very much more 

 compact form, will be found amongst those exhibited, and also 

 other reflectors made by Sir William Herschel, the late Lord 



