134 



ACCBSSOEY APPARATUS. 



age from the surface of the glass in front of it. This secondary 

 image, it has been thought, interferes with the perfect perform- 

 ance of the achromatic condenser; and hence, for obtaining the 



Fig. 37. 



Powell and Lealand's Achromatic Condenser. 



most satisfactory definition, some Microscopists prefer to direct 

 the axis of the microscope to the source of light (the mirror being 

 turned aside) ; whilst others, feeling the inconvenience of the 

 position thus required, have recourse to a prism which shall give 

 the required reflection with only a single image. The prism 

 iisually employed (having been originally applied to this purpose 

 by M. Dujardin) has plane surfaces, and acts, therefore, as the 

 equivalent of a plane mirror. A reflecting prism has been de- 

 vised, however, by Mr. Abraham (optician of Liverpool), which 

 is intended by him to take the place both of mirror and achro- 

 matic condenser, though its action (as it seems to the author) 

 must rather be that of the ordinary concave mirror ; this has one 

 of its surfaces hollowed out to receive one side of a double-con^ 

 vex lens, the other side of which acts as the emergent surface of 

 the prism, causing the rays as they pass through it to converge ; 

 and the prism itself being composed of flint-glass, whilst the lens 

 is of crown, no chromatic dispersion of the rays is produced, 

 though the spherical aberration is not coi'rected. 



58. White-iJloud Illuminators. — It being universally admitted 

 that the light of a bright white cloud is the best of all kinfls of 

 illumination for nearlj^ every kind of Microscopic inquiry, various 

 attempts have been made to obtain such light, from the direct 

 rays either of the sun or of a lamp, by what may be called an 

 artificial cloud. Some have replaced the plane mirror by a sur- 

 face of pounded glass or of cai'bonate of soda, or (more com- 

 monly) by a disk of plaster of Paris, the latter being decidedly 

 the preferable method ; but a sufficiently bright light is not thus 

 obtained, unless a condenser be employed to intensify the illumi- 

 nation of the mirror. Such a condenser may be most conveni- 



