90 



require a long and patient experimental inquiry, for which I have 

 not yet had leisure, but I give my theory in its crude state, with the 

 view of inducing others to follow the same path, and try its effect by 

 different means, for I must observe that I adopted a truncated 

 reflector, placed under the object, as the readiest means of carrying 

 out the theory, and obtaining the purest light and utmost limits of 

 aperture ; and if this effect is obtained, either by reflection or refrac- 

 tion, and all the niceties of the principle attended to, it will be found 

 a valuable adjunct to microscopic discovery, for from appearances I 

 have observed with it, it is my belief that, on account of the defects 

 of direct transmitted light which I have attempted to explain, many 

 transparent objects, from the earliest age of the microscope till now, 

 have been seen under false appearances. This may appear a pre- 

 sumptuous assertion, but I confidently trust to time to prove its 

 accuracy. 



EXPLANATION OF PLATE XIV. 



Fig. 1. An arrangement for obtaining a large angular pencil of achromatic light, 



reflected obliquely across the object in one direction. 

 Fig. 2. Two of Nachet's prisms, placed so as to throw two equal pencils of oblique 



light across the object from opposite directions, in order to balance the 



defects arising from such illumination from one side only. 

 Fig. 3. A truncated parabolic reflector for condensing lateral light upon transparent 



objects ; all rays from the source of light are prevented from entering the 



object-glass, and a black field is obtained by means of an adjusting dark 



well, placed in the centre of the reflector, and supported by a glass plate, 



BB. 

 Fig. 4. Kepresents the aberrations produced on the large angular pencil of rays from 



the reflector by passing through the plate of glass upon which objects are 



mounted. 

 Fig. 5. Shows the method of correcting the aberrations. The rays from the reflector 



are refracted by the concave surface of the meniscus, A A., placed at the 



apex, and after passing through the glass slide, FF., are brought to a 



point. 

 Fig. 6, AA. The meniscus lowered from the position shown in fig. 5 ; the rays being 



refracted by both surfaces, are brought to a focus. 



