ISO 



SCIENTIFIC RECREATIONS. 



Fig. 156. Concave lens. 



something about 



the convex lens, near which an object, PQ, is placed a little beyond its focal 

 length. An inverted image, pq, will then be formed. This image is pro- 

 duced in the convex lens, bab\ and when the rays are 

 reflected out they are parallel, and are distinctly seen. So 

 the eye of the observer at the point E will see a magnified 

 image of the object at PQ brought up to/^ (fig. 155). 



Sir Isaac Newton suggested the Reflecting Microscope, 

 and Dr. Wollaston and Sir David Brewster improved the 

 instrument called the " Periscopic Microscope," in which 

 two hemispherical lenses were cemented together by the 

 plane surfaces, and having a "stop" between them to limit 

 the aperture. Then 

 the "Achromatic" in- 

 strument came into 

 use, and since then 

 the Microscope has 

 gradually attained 

 perfection. 



We have so fre- 

 quently mentioned 

 lenses that it may be as well to say 

 them. Lenses may be. spherical, double-convex, plane- 

 TlS ' I55 'Te^e e s M ' lcr SCOpe convex > plane-concave, double-concave, and concave-convex. 

 Convex lenses bring the parallel lines which strike them to 

 a focus, as we see in the "burning-glass." The concave or hollow lens appears 

 as in fig. 156. The rays that follow it parallel to its axis are refracted, and 

 as if they came from a point F in the diagram. But converging rays falling 

 on it emerge in a parallel direction as above, or diverge as in fig. I 5 8. 



The use of spectacles to long or short-sighted people is a necessity, 

 and the lenses used vary. The eye has usually the capacity of suiting 



itself to viewing objects its ac- 

 commodation, as it is termed 

 near or far. But when the fore- 

 part of the eye is curved, and 

 cannot adapt itself to distant 

 objects, the person is said to be 

 short-sighted. In long sight the 

 axis of the eyeball is too short, 

 and the focus falls beyond the 

 retina ; in short sight it is too 

 Fig. 157. long. In the diagrams herewith 



i. Focus of parallel rays. 2. Focus of divergent rays. fig. 159 shoWS by the dotted 



3. Focus of divergent rays brought forward by more convex lens. .. . . _ , . 



lines the position of the retina in 



long sight, and fig. 160 in short sight, the clear lines showing in each 

 case , the perfectly-formed eye. For long sight and old sight the double- 



