THE TELESCOPE. 



149 



telescopes were made with the concave eye-lens. Rheita, a monk, made a 

 binocular telescope, as now used in our opera- and field-glasses approximately. 



But the prismatic colours which showed themselves in the early tele- 

 scopes were not got rid of, nor was it till 1729 that Hall, by studying the 

 mechanism of the eye, managed a combination of lenses free from colour. 

 Ten years before (in 171 8) Hadley had established the Reflector Telescope ; 

 Herschel made his celebrated forty-foot "reflector" in 1789. 



However, to resume. In 1 747, Euler declared 

 that it was quite possible to construct an arrange- 

 ment of lenses so as to obtain a colourless image, 

 but he was at first challenged by John Dollond. 

 The latter, however, was afterwards induced to 

 make experiments with prisms of crown and flint 

 glass. He then tried lenses, and with a concave 

 lens of flint, and a convex lens of crown, he 

 corrected the colours. The question of proper 

 curvature was finally settled, and the " Achromatic" 

 Telescope became an accomplished fact. 



There are two classes of Telescopes the 

 reflecting and refracting. Lord Rosse's is an 

 instance of the former. Mr. Grubb's immense 

 instrument in Dublin is a refractor. 



The Microscope has been also attributed to Fi s- ' 53 The Microscope. 

 Zacharias Jansen, and Drebbel, in 1619, possessed the instrument in London, 

 but it was of little or no use. The lens invented by Hall, as already 

 mentioned, gave an impetus to the Microscope. In the simple Microscope 

 the objects are seen directly through the lens or lenses acting as one. The 

 compound instrument is composed of two lenses (or a number formed to do 

 duty as two), an eye-lens, and an object-lens. Between these is a "stop" to 

 restrain all light, except what is necessary to view the object distinctly. 

 The large glass near the object bends the rays 

 on to the eyeglass, and a perfect magnified 

 image is perceived. We annex diagrams, 

 from which the construction will be readily 

 understood. 



We have in the previous chapter men- 

 tioned the effect of light upon the eye and 

 its direction, and when an object is placed 

 very near the eye we know it cannot be 

 distinctly seen ; a magnified image is thrown 

 upon the retina, and the divergency of the rays prevents a clear image 

 being perceived. But if a small lens of a short " focal length " be placed 

 in front of the eye, having PQ for its focus, the rays of light will be parallel, 

 or very nearly so, and will as such produce " distinct vision," and the image 

 will be magnified a.\. pq. In the Compound Refracting Microscope, BAB is 



Fig. 154. Image on the Retina. 



