MEMORANDA. 161 



merely because they form no representative of the conditions 

 of an object, mounted in the substance of a refracting medium. 



Mr. Sollitt imagines that the theory, as I have explained it, 

 is tantamount to this, — that when a parallel plate of a refract- 

 ing body is interposed between the object-glass to be mea- 

 sured and the candle, it should reduce the aperture. How 

 he could interpret my meaning thus I am at a loss to conjec- 

 ture, when I stated distinctly, in my paper in the ' Micro- 

 scopical Journal,' page 213 : — "That a parallel plate of glass, 

 over an object mounted dry, has no effect in reducing the 

 aperture, for the rays, after being deflected by the first sur- 

 face, emerge again from the second one, parallel to their 

 original direction, and all converge to a point, at the same 

 angle as at first ; consequently, the object is seen through the 

 glass, with an angle of aperture the same as if it was not 

 interposed." This is also the case, if several parallel plates 

 of substances of different refractive powers be interposed. 



That the candles which Mr, SoUitt made use of exactly 

 represented objects mounted dry, there can be no question ; 

 and his results are as they should be. But if each candle 

 had been a body contained in the substance of a solid block 

 of glass, extending from the object-glass ; then a very different 

 indication, or a diminished aperture, would have been ob- 

 tained. 



I imagined that my explanation would have been very 

 easily understood, considering that it strictly depended upon 

 the first law of refraction, contained in any elementary work 

 upon optics. The annexed cut may serve as a particular 

 illustration of the method of measuring the loss of aperture 

 on objects in transparent media. In diagram 1, let a a repre- 

 sent a parallel plate of glass, or other medium. Suppose an 

 object to be immersed just within its substance at b, to be 

 viewed with an object-glass, whose aperture is c c. Now, 

 according to the laws of refraction, the aperture, or angle of 

 rays, collected from the object must be represented by the 

 lines taken from b to the points of incidence of the exterior 

 rays of the object-glass, at the upper surface of a a. This is 

 the theoretical explanation of the loss of aperture on an object 

 thus mounted. 



The practical method of obtaining the actual measurement 

 of the refracted and reduced angle of aperture is demonstrated 

 by diagram 2. Let c c again represent the aperture of the 

 object-glass, which is focussed exactly on to the upper surface 

 of aa ; the rays, cc, will be refracted to d d, forming an angle 

 exactly similar to that at diagram 1, but inverted. It is 

 therefore evident, that if we measure the base or diameter 



VOL. III. M 



