80LLITTS ILLUMINATION. 



175 



" My first attempts at oblique prism-illumination were 

 made, many years ago, with Abraham's (of Liverpool) 

 Achromatic Lenticular Prism ; the high character given 

 of the performance of this instrument by the late Professor 

 Quekett and by Mr. Sollitt, of Hull, led me to believe 

 that it would entirely supersede the necessity of using a 

 condenser during the examination of the highest class 

 test-objects. In my hands, however, it failed to do this ; 

 the pencil of light transmitted proved to be too diffused, 

 and the shadows too faint to enable me to bring out the 

 markings of single-lined tests satisfactorily. I subjoin Mr. 

 Sollitt's directions for using this prism, as communicated 



Fia. 102 



■Sollitt's Single-Prism Illumination. 



to Mr. Abraham : — ' "When I require the prism for defi- 

 nition and direct light, I use it in place of the mirror 

 under the stage ; but when required for oblique light, that 

 is, for illuminating lined objects, or for black-ground 

 illumination, I use it on a separate stand, as shown in 

 fig. 102, the object being illuminated by the very oblique 

 ray, c B. Let a b be a supposed ray of direct light, coin- 

 cident with the axis of the microscope, then so long a3 the 

 angle u B a is less than half the angle of aperture of the 

 object lenses, the field will remain illuminated by oblique 

 light, and the lined objects (if placed in their proper 

 position) will be brought out in the most beautiful manner. 

 I place the prism about five inches in front of the object, 

 and by this method, with my widest apertures, I bring 



