SUBSTAGE CONDENSERS, ETC. 97 



3. To ascertain the amount or rather the size of the aplanatic cone is quite another 

 matter. First, what is meant by an aplanatic cone? It is a term often used but 

 rarely explained on account of the somewhat involved nature of the reply. The 

 word itself, derived from the Greek, means, in point of fact, " free from wandering," 

 by which the optician understands (as he uses the word) that all rays, whether from 

 the periphery of the lens or nearer its axis, shall meet in one point in a given 

 plane, as shown in Eig. 49. This is the ideal perfection of the optician's art. 

 An^ ordinary uncorrected lens will always suffer from what is called spherical aber- 

 ration, by which is meant the marginal rays come to a focus in a point on the axis 



Fig. 49 



Fig. 50 



Uncorrected Lens 



Fig. 52 



Over Corrected 



Under Corrected 



closer to the lens than those situated nearer to its axis or centre, as seen exaggerated 

 in Fig. 50. The art referred to of the optician is to try and unite these planes of 

 focus by combining glasses having different properties. If he overdoes it, producing 

 what is technically called " over-correction," he brings the peripheral rays too far along 

 the axis, as shown in Fig 51, and if he does not correct enough— " under correction" 

 as it IS termed— he leaves the combination with the same error outstanding, 

 although to a less degree, as that possessed by an uncorrected lens, as shown (greatly 

 exaggerated) in Fig. 52. 



Now in a condenser it is evident spherical aberration should be as perfectly 

 corrected as possible, and the ideal form should transmit a cone of rays of uniform 

 light equal to its full aperture. In other words, its spherical aberration should be 

 m7. Few condensers indeed will approximately do this. Even those by Zeiss are 

 greatly faulty in this respect, but the last dry apochromatic by Powell and Lealaud 

 IS a decided advance. The best series we have ever seen, we feel bound to admit, are 



