OP THE MICROSCOPE. 157 



may be either the light from a bright cloud thrown up by the 

 plane mirror ; divergent rays from a lamp made parallel with 

 the concave mirror ; or divergent rays from a lamp made 

 parallel with a bulls-eye condenser, and thrown up by the 

 plane mirror ; or a combination of these last. Thirdly, we must 

 remember that the instrument has two centres, arising from the 

 fact that the optic axis is bent by internal reflection from 

 the polished facet of the cylinder. Hence, we have the centre of 

 the lens below the cylinder, to which the illumination must be 

 adjusted, and we have another centre at which the rays are 

 brought to a focus, and which must be made to coincide with 

 the optic axis of the microscope. This centre is generally 

 marked by the maker on the small side piece, e, and it should 

 be brought exactly to the centre of the field of view of the ob- 

 jective, a low powqr say a half inch or two-thirds being use.l 

 for this purpose, in the first place. 



It is. of course, obvious that by adjusting the other condi- 

 tions of the instrument, the points which we have laid down 

 may be varied. Thus, for example, divergent rays may be 

 used ; but in that case the focus of the rays will not be at the 

 point, g. unless the flat top of the cylinder is depressed a little, 

 which may be easily done if a tenacious liquid like glycerine be 

 used as the connecting medium. 



We think that if the reader will bear these points in mind, 

 and will work over this accessory faithfully for a few hours, he 

 will attain results which will abundantly compensate him for 

 the labor spent. 



The reader must always bear in mind that where totally re- 

 flecting surfaces are used, as in paraboloids, reflex illuminators, 

 etc., they must be kept scrupulously clean or they lose their 

 effect. 



The Wenham Prism. This simple and ingenious little 

 device was first described by Mr. Wenham in a paper read 

 before the Eoyal Microscopical Society, March 26, 1856. At 

 that time immersion objectives were unknown, or at least were 

 not in use, and the effect of the prism was to produce a very 

 brilliant dark-ground illumination ; but, as with the Keflex 

 Illuminator, the introduction of immersion objectives of wide 



