274 PKACTICAL PHOTO-MICROGKAPHY 



starch. This does away with the reflecting surface, and may be 

 washed off again immediately the photograph has been taken, 

 without any harm, as a rule, being done to the specimen. 



Stereoscopic Photo - micrography. The production of 

 stereoscopic photo-micrographs is almost entirely confined to 

 low-power work, and the best results are to be obtained with 

 objectives lower than one inch. The process must not be 

 confused with ordinary binocular microscopic vision. It consists 

 essentially of obtaining two photographs side by side on one 

 plate from different points of view in relation to the object, or 

 by means of different zones of the objective. It is also depen- 

 dent on getting more than one plane of the object in focus ; 

 in fact, as it depends essentially on viewing the object in three 

 dimensions, the focal length of the objective must be consider- 

 able ; in addition, means are generally taken to restrict the 

 aperture of the objective as far as is optically practicable, to 

 obtain the utmost penetration. 



The apparatus in general need not differ from any one 

 already described, except that the dark-slide may be sufficiently 

 large to enable two photographs to be taken on the same plate. 

 There are three methods at present in general use. The first 

 and simplest is to cover up one half of the back combination of 

 the objective by means of a stop ; an appliance to do this consists 

 of a short fitting which screws in between the objective and the 

 nose-piece of the microscope. A slot is cut in this fitting, into 

 which is dropped a blackened brass slip, so that exactly one half 

 of the back combination of the objective is covered. One 

 negative is taken through the uncovered half, and then the stop 

 is transferred to the opposite side, and another negative is again 

 taken on the same plate. Prints from these negatives, when 

 suitably mounted, yield a satisfactory stereoscopic effect. 



Instead of covering and uncovering the half of the objective, 

 a better method is to place in the same position stops with 

 circular openings. The advantage of this method over the one 

 just described is, as pointed out by Mr. J. Kheinberg, 1 that 

 owing to the shape of the diffusion-discs in those parts of the 

 object which are not in exact focus being dependent on the 

 shape of the stop at the back of the objective, the diffusion- 

 discs are at their best with circular openings. There is the 



1 Journal of the Queckett Microscopical Club, 1906, pp. 371-396, 



