1886-87.] ApochroDiatic Object-G lasses. 21 



y.~APOCHROMATIC OBJECT-GLASSES. 



By Mr Wm. FORGAN. 



{Read Dec. 22, 1886.) 



What is an apochromatic object-glass, and in what respects 

 is it better than an ordinary achromatic one ? The term 

 apochromatic means " free from colour." Now we have 

 hitherto always regarded an achromatic lens as essentially 

 possessing this property of freedom from colour. To enable 

 one to understand the matter properly, it is necessary to begin 

 with an illustration of what takes place when light is trans- 

 mitted through and refracted by an ordinary convex lens. Light 

 so refracted is bent or brought to a focus behind the lens at a 

 point nearer to or farther away from it, depending upon the ex- 

 tent of its curvature. But from the fact that the separate rays 

 of light have not all the same refractive power — those at the 

 violet end of the spectrum having the greatest and those at 

 the red end the least — the coloured rays into which the light 

 is broken up by the lens do not form the focal point at one 

 and the same distance, but each ray has a focus of its own, 

 the consequence being that the focal point of the violet ray is 

 nearest to the lens, and the focal point of the red ray farthest 

 away. It may be readily seen from this that every ordinary 

 lens has as many focal points as there are rays in the 

 spectrum. Wlien the principle of achromatism was discovered, 

 it was found to be possible, by using glass of different 

 densities, to combine two of these rays, and so render the 

 images produced by such a lens what was termed achromatic — 

 the two rays selected for combination being those which gave, 

 by means of the glass, the whitest and purest image. In this 

 case all the other colours were left necessarily out of view, 

 and, so to speak, to shift for themselves. These outstanding 

 or uncorrected rays are what is called the " secondary 

 spectrum." The combination of the two primary rays by the 

 lens rendered the others so little conspicuous, except in certain 

 cases, that they had to be, and were, necessarily disregarded. 



Another point in regard to this so-called achromatic com- 

 bination, and a very important one, is this, that as every ray 



