5° 



THE MICROSCOPE. 



objective. If the resolution were done by central light, the light 

 would be seen through the center of the back lens, by looking down 

 the tube; while if oblique light were the cause of it, the light would 

 be seen through the edge of it. 



It is well known that an objective which is properly corrected 

 (chromatically) by oblique light, with any given immersion fluid, 

 will be decidedly undercorrected by central light; and if properly 

 corrected by central light, it will be overcorrected by oblique light. 

 Thus it can be determined whether the resolution is accomplished 

 by central or oblique light; and I think that if Prof. Forbes and 

 Dr. Detmers had noticed the correction they would have found that 

 it indicated central light. 



But the question arises: °,U the resolution be by oblique light, 

 how does the light enter the objective, when the mirror is central ? 

 That Dr. Detmers is wrong in supposing the homogeneous fluid to 

 act as a lens and thereby illuminate the object, I think will be evi- 

 dent to any one who will carefully try the experiment. And in the 

 solution given by Prof. Forbes, viz., that of the reflections between 

 the top and bottom of the slide, I think it would be found that in- 

 sufficient light would be the result. 



The real explanation is this: The light from the mirror (cen- 

 tral) is condensed on the edge of the front cell of the objective. 



