OBJECT GLASSES AND EYEPIECES 



85 



In this figure the lines at C and F coincide, but those at 

 D and G do not, and the want of coincidence at D and G gives an 

 idea of the secondary error. 



If we make a pair of lenses out of these two glasses which 



when combined together give an achromatic lens of 1 inch focus, 



and correct for C and F, we find that the foci for the different 



rays are : 



C 1-00000 



D -99963 



E 1-00000 



G 1-00165 



In all kinds of optical glass with high dispersion the relative 

 dispersion from F to G is higher than those of low dispersion, 

 but in some the difference is slight, and telescope lenses with 

 reduced secondary spectrum can be made from these. By com- 

 bining three glasses together apochromatic telescope object glasses 

 can be made, but in these the lenses have to be of relatively 

 short focus, and consequently only small apertures compared to a 

 microscope object glass can be obtained; //6 is a very large aperture 

 for a telescope object glass, but only corresponds to '07 N.A. 

 in a microscope object glass. No apochromatic microscope 

 object glasses have yet been made satisfactorily by this means. 



The peculiar mineral fluorspar, however, has totally different 

 properties. It has a low refractive index and an extremely low 

 dispersion about 1/95 on the focus. The great peculiarity, 

 however, is that it has a larger proportional dispersion from 

 F to G than many of the glasses with higher dispersion ; the 

 corresponding figure being 583. Now if we make a similar diagram 

 to the last, but of fluorspar and light baryta flint, we get : 



-to 



D 



-to- 



F- 



-to — G 



Fluorspar 



Light baryta flint 



A combination made of these two materials and achromatic, of 

 1-inch focus, gives the following results for foci of different colours : 



C 

 D 

 F 

 G 



1-00000 



1-00000 



1-00000 



-99947 



Now it is quite evident that combining with this combination 

 another combination such as the first, in which the aberration for 

 F to G is in the opposite direction, it is possible to produce a lens 

 in which all the foci for the four rays are the same. 



