64 PKACTICAL PHOTO-MICBOGBAPHY 



is the parabolic side-silver-reflector. It consists merely of a 

 concave piece of metal silvered on its inner side, and this is so 

 placed above the object that a beam projected by the bull's- 

 eye condenser is condensed by the internal reflecting surface 

 of the reflector, and the light thus thrown on to the object. It 

 may be attached either to the microscope body or placed on 

 the stage, and is usually supplied with a ball-and-socket joint. 

 The advantage of this is that the illumination, although still 

 confined to low powers, is not at such an acute angle to the 

 surface of the object as when the bull's-eye alone is used. 



A further development of this method is that known 'as 

 the Sorby parabolic reflector. It consists of a parabolic 

 surface with an auxiliary reflector, the latter arranged so 

 that it can be turned out of action if desired. This reflector 

 is suitable for powers not higher than one inch, and is made to 

 slide on to the mount of the objective. It is an improvement, 

 and gives better illumination than the simple side-reflector 

 already described, and is particularly applicable to metal- 

 lurgical work. With both these side-reflectors a bull's-eye 

 must be used to throw a parallel beam on to the reflecting 

 surface, and it is essential to see that the projected parallel 

 beam illuminates the whole of the reflector. The position of 

 the radiant should be such that the smallest possible image is 

 thrown on to the surface to be illuminated. The best results 

 are obtained when the reflector is specially made for the 

 particular objective in use a very brilliant image then 

 resulting. It throws light upon the object over nearly 180 

 of arc out of a possible 360, and thus illuminates a consider- 

 able amount of a solid object at one time, and lessens to a 

 great extent the shadows that are obtained by the method 

 in which the bull's-eye condenser alone is used. If it is 

 made for and mounted on the objective, it is simple in use 

 as no adjustment is required ; but where only one reflector 

 is used for lenses of varying focal lengths, it is necessary 

 for it to be so arranged that it slides up and down on the 

 objective until correct adjustment is obtained. 



With the Sorby parabolic reflector the flat silvered mirror, 

 which swings in and out as described, results in the light being 

 thrown down directly on to the object, and thus a close ap- 

 proximation to vertical illumination is obtained. The mirror 



