424 Dr. Goring on Achromatic Microscopes, 



an enormous angle of aperture — none of the achroma- 

 tics singly exhibit them satisfactorily. Mr. Lister's 0*3 

 does more with them than any other; but it may be 

 considered lame and impotent, and rather giving indica- 

 tions of the lines than showing them outright^. When 

 we reflect that they are not to be made out by single lenses, 

 unless their aperture is somewhere about equal to their focus, 

 it is surprising that the achroraatics should show so much of 

 them as they do, with an aperture not more than equal to 

 half their focus. An Amicican reflector, having an object 

 metal of 0*3 focus with 0*2 of aperture, shows them com- 

 pletely well, with the cross fibres much more numerous than 

 in the plate ; and one of 0*6 focus with 0*3 of aperture, with 

 a very oblique illumination, gives very strong indications of 

 them, if it does not actually demonstrate them fairly drawn 

 out. As to the reason why this class of objects should re- 

 quire such enormous angles of aperture to resolve their lines, 

 I must say I am completely mystified. It is easy enough to 

 show why nebulae and clusters of stars should require an im- 

 mense aperture to render them visible ; but why mere lines 

 extremely similar to those on micrometers (if we may judge by 

 their prismatic effect and general appearance) should require 

 such vast calibres, when the artificial ones are in no such pre- 

 dicament, must be admitted to be very perplexing : — the only 

 property they seem to possess in common, is that of being 

 best seen with an oblique light : the lines on glass microme- 

 ters, viewed as transparent objects, do not indeed totally dis- 

 appear in a direct light through the axis of the microscope, 

 but are infinitely better seen when it comes askance. 



Fig. 6, a and b, are two more feathers which seem to be 

 in the same condition with the last, as far as regards the aper- 

 ture required to show them : they were procured from the 



* I have seen them indifferently well with a 0*3, which was in a very 

 imperfect state, and scarcely fit to act upon opaque objects at all. By com- 

 bining two aplanatics together, a sufficient aperture is obtained to bring 

 them out ; but it is a way of effecting the purpose which ought to be 

 avoided if possible. Mons. Chevalier combines two of his cemented 

 double object-glasses together, which in my opinion form the best possi- 

 ble combination of this sort, giving a much more brilliant image than 

 triple ones treated in the same way, which, from the number of their 

 surfaces invariably produce great dulness in the image. 



