14:8 .SELECTION AND trsB 



denser, the direction and extent of these shadows may be greatly 

 varied. Hence one of the advantages of the students' lamp. 



An important use of the condensing lens is to change the 

 direction or character of the rays employed. Thus, when a 

 lamp is in use the rays are divergent, and the easiest way to 

 render them parallel is to pass them through a condensing lens. 

 To effect this the distance of the lens from the lamp must be 

 exactly the same as that at which it brings parallel rays to a 

 focus. In other words, the lens must be at a distance from the 

 lamp which is exactly equal to its focal distance for parallel rays. 



Condensing lenses are made of all sizes, and some of them 

 are quite expensive, but we have frequently obtained wonder- 

 fully fine results by means of a cheap lens of small size, but 

 good form. A condensing lens is, perhaps, the most important 

 accessory that can accompany a microscope. 



Side Reflector. This is a small silvered concave mirror, 

 which is used to throw the light on the object for the same pur- 

 pose as the condensing lens. The results which it gives are 

 slightly different, and it is a most valuable means of illumina- 

 tion. It has not been so generally introduced as it deserves to 

 be, and few microscopes are furnished with it unless to special 

 order. It should always be used in combination with a bulls- eye 

 condenser, as light of much greater intensity is thus obtained. 



Tlie Liefoerk-iilm. This was one of the first instruments 

 used for illuminating opaque objects. It consists of a small, 

 concave, spherical mirror, through the centre of which the ob- 

 jective passes, the focus of the mirror and objective coinciding. 

 The object must be small, and is generally mounted on a small 

 circular disc of leather or card, which stops out the central 

 rays, while the light which passes round it strikes against the 

 concave mirror, and is reflected back again upon the object. 



The Lieberkuhn gives very brilliant effects with many ob- 

 jects, and if well managed it enables us to obtain very satisfac- 

 tory views, especially with powers which are too high to admit 

 the use of the side reflector, the parabola, or the bulls-eye. 

 The great objection, however, is the fact that the light falls 

 almost vertically, thus obliterating all the shadows pro- 



