726 SUMMARY OF CURRENT RESEARCHES RELATING TO 



obtain ribbons more or less incurved owing to want of perfect recti- 

 liniarity in the preparation of the paraffin block. The author has sought 

 to regularise the inrolling so as to form ribbons of sections whose 

 curvature should vary with the circumference of the disk where they are 

 to be deposited, with the dimensions of the block to be sectionised. To 

 obtain this result it is only necessary to give to the paraffin block a shape 

 whose two faces correspond to two rays of the disk determined by the 

 dimensions of the object. This cutting is facilitated by the use of a 

 specially designed articulated and adjustable knife. A Minot-Zimmermann 

 microtome with vertical carrier is adapted so that the sections fall, as 

 cut, on a rotating glass disk and range themselves spirally, ready, save for 

 clearing and covering, for immediate examination. 



4. Rotatory Stage. — The disk just referred to, is of a size suitable for 

 the Microscope stage, and is transferred to it. The disk is perforated at 

 its centre, and this perforation enables it to be placed on a sort of 

 vertical pin in the centre of the stage and secured by a nut. A rack- 

 work movement is connected with the pin so that the disk may be 

 rotated or moved as a whole in two mutually perpendicular directions. 



Mounting Stereoscopic Views.* — Stereoscopic slides cannot give a 

 true stereoscopic effect unless they are observed under the same con- 

 ditions of convergence as those which prevailed when the exposures 

 were made, and the true effect cannot be secured unless the views are 

 mounted with the proper degree of separation, and observed from the 

 proper distance. If the two negatives are on one plate, as is usual in 

 most cases, and are produced with lenses a known distance apart, it is 

 quite easy to ascertain the proper mounting separation for the positives. 



The complete rule for a lenticular stereoscope is as follows : — Add 

 the separation between the two lenses (centre to centre) to the distance 

 separating the eyes of the observer, and then deduct the distance 

 between any two corresponding points on the two negatives. The 

 result is the proper separation of the same two points in the positive 

 prints. If the points selected are distant points, their separation on 

 the negatives will be equal to the lens separation, and the two distances 

 will therefore cancel each other. It is then only necessary to mount 

 the positives so that two corresponding distant points are separated by a 

 distance equal to that between the eyes. If there are no distant points 

 in the view, the complete rule must be applied, and if all the objects 

 are very near, it is especially important that the rule should be strictly 

 observed. If a prismatic stereoscope is to be used, the width of one 

 prism should be added to the positive separation as found by the rule. 

 This addition is exactly correct if the prisms are half -lenses properly 

 centred in front of the eyes ; and though these conditions are seldom 

 fulfilled, the correction is generally true enough for all practical pur- 

 poses. With very near objects and widely separated lenses the proper 

 separation for the positives is sometimes so small that the prints have to 

 be trimmed down to absurdly small dimensions. Further, the negative 

 images come very near the ends of the plate. To avoid these effects 

 the separation of the lenses must be reduced, and the adjustment is 



* British Journal of Photography, See also English Mechanic, lxxxiv. (1906) 

 p. 208. 



