SOME SPECIAL PKOCESSES 275 



further advantage that both the size and separation of the 

 openings may be varied to suit the particular objects, or to 

 increase or decrease the amount of desirable penetration. The 

 procedure is precisely the same as before, the stop being placed 

 at one side of the objective and then removed to exactly the 

 opposite side to take the succeeding photograph. It is obviously 

 not a matter of moment whether, in relation to the position of 

 the microscope, the stops are side by side or above and below ; 

 this will depend simply on the position of the plate in relation 

 to the apparatus. 



Mr. H. Taverner A mentions that the size of the circular 

 openings may vary between 2J mm. and 4 mm., and that the 

 inner edge of the aperture will not generally require to be more 

 than 1 mm. from the optic axis of the objective. The size 

 of stop should be the largest possible to give the necessary 

 penetration and to bring out detail sharply in the picture. 

 The inner edge of the aperture should be as near the centre of 

 the objective as will give the necessary stereoscopic effect, while 

 at the same time giving the greatest impression of depth. 

 These circular stops are carried in a suitable fitting at the back 

 of the objective, and may be obtained from Messrs. Baker of 

 High Holborn. Mr. Taverner also recommends the use of a 

 repeating back instead of an ordinary dark-slide. The advan- 

 tage of this is that when prints are taken from the pictures they 

 are in their proper position relatively to one another for use 

 in a stereoscope that is, they do not require cutting and 

 remounting. It is important, whichever method is adopted, 

 to see that the apertures are in exact alignment when changed 

 from side to side ; otherwise the resulting pictures will not 

 superimpose properly to give the necessary stereoscopic effect. 



A third method of obtaining the same result is by shifting 

 the objective or the object by a definite amount from side to 

 side. This is the method advocated by Mr. A. C. Banfield.- 

 The principle involved is that the inter-ocular distance, normally 

 62 mm., must be divided by the required magnification to 

 give the angular separation through which the objective or the 

 object must be moved. Thus for 32 diameters the separation 



1 Journal of the Royal Microscopical Society, 1906, p. 260. 



2 Journal of the Queckett Microscopical Club, 1909, pp. 459-464 ; Journal of 

 the Royal Microscopical Society, 1910, p. 233. 



T 2 



