WITH THE MICROSCOPE. 257 



should pass through the object, it being indifferent in what direction 

 this motion is made in respect to the stand." He proposes also a 

 simpler method, which is, to make the object itself partly revolve 

 round an imaginary axis within itself, from 7 to 15, care being taken 

 to render the illuminations equal, and avoid interfering shadow's, so as 

 not to produce pseudoscopic effects. 



Mr. Wenham showed that images of objects could be produced 

 with such a difference in the relative position of their parts when 

 viewed, by stopping off the alternate halves of the object-glass, or the 

 emerging pencils from the opposite halves of the eye-piece, that these 

 images when recombined had a perfect stereoscopic character. 



To effect this, Mr. Wenham's plan was, to place a sliding stop 

 with straight edges against the lens of the objective, so that it could 

 be turned to cut off either the right or left portion of the lens ; he 

 found those of large angular aperture would need only one-third of 

 their diameter stopped off. 



Professor Riddle, of New Orleans, proposes to accomplish the 

 same end by inserting, just behind the object-glass, a small equilateral 

 prism, arranged on a central axis parallel to its polished faces and 

 transverse to the axis of the object-glass, so that it can be inclined. 

 The hypothenuse being placed coincident with the axis of the micro- 

 scope, on making the necessary inclination it will furnish the appear- 

 ance of the object itself being moved, and when the image of the 

 object has been drawn with the prism in one position it is to be 

 altered slightly, and the slide moved so as to bring the same part 

 into the centre of the field of view, as at first ; it will now have an 

 altered aspect, corresponding to the difference in point of view, 

 equivalent to the number of degrees of the inclination of the prism 

 which may vary from 4 to 9. And if the two images of such a 

 drawing, or photographic impression, be viewed stereoscopically, 

 they will be found to coalesce into a stereoscopic image. 



Mr. Heisch, in October, 1862, recommended, as an adapter for 

 the object-glass, one carrying a tube that can be turned half round 

 by a lever outside. In this tube is another, provided with a stop, 

 that cuts off half the pencil of light emerging from the object-glass ; 

 this sliding tube, when placed in proximity to the back lens of the 

 objective, is so arranged that the field on the ground glass of the 

 camera shall be equally illuminated in all positions of the stop. 

 The image is thrown on a prepared sensitised plate for the first 

 picture, the stop is then turned round until it stands in a direction 

 opposite to the first position ; the unimpressed half of the prepared 

 plate is then shifted into the field, and in its turn receives the second 

 image. The two resulting pictures furnish a stereoscopic effect. 



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