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IHE POPULAR SCIEJ^CE MONTHLY. 





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Fig. 3. Achromatic Object Glass 

 a, crown glass ; 6, flint glass. 



which its construction depends is that by combining lenses of 

 different dispersive power the dispersion of the spectral colors 

 can be corrected while the convergence of the rays of light toward 



a focus is not destroyed. Flint 

 glass effects a greater dispersion 

 than crown glass nearly in the 

 ratio of three to two. The chro- 

 matic combination consists of a 

 convex lens of crown backed by 

 a concave, or plano-concave, lens 

 of flint. When these two lenses 

 are made of focal lengths which 

 are directly proportional to their 

 dispersions, they give a practical- 

 ly colorless image at their com- 

 mon focus. The skill of the tele- 

 scope-maker and the excellence 

 of his work depend upon his se- 

 lection of the glasses to be combined and his manipulation of the 

 curves of the lenses. 



Now, the reader may ask, " Since reflectors require no correc- 

 tion for color dispersion, while that correction is only approxi- 

 mately effected by the combination of two kinds of lenses and two 

 kinds of glass in a refractor, why is not the reflector preferable 

 to the refractor ? " 



The answer is, that the refractor gives more light and better 

 definition. It is superior in the first respect because a lens trans- 

 mits more light than a mirror reflects. Prof. Young has re- 

 marked that about eighty-two per cent of the light reaches the 

 eye in a good refractor, while " in a Newtonian reflector, in aver- 

 age condition, the percentage seldom exceeds fifty per cent, and 

 more frequently is lower than higher." The superiority of the 

 refractor in regard to definition arises from the fact that any dis- 

 tortion at the surface of a mirror affects the direction of a ray of 

 light three time as much as the same distortion would do at the 

 surface of a lens. And this applies equally both to permanent 

 errors of curvature and to temporary distortions produced by 

 strains and by inequality of temperature. The perfect achroma- 

 tism of a reflector is, of course, a great advantage, but the chro- 

 matic aberration of refractors is now so well corrected that their 

 inferiority in that respect may be disregarded. It must be ad- 

 mitted that reflectors are cheaper and easier to make, but, on the 

 other hand, they require more care, and their mirrors frequently 

 need resilvering, while an object glass with reasonable care never 

 gets seriously out of order, and will last for many a lifetime. 

 Enough has now, perhaps, been said about the respective 



