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SUMMARY OF CURRENT RESEARCHES RELATING TO 



I 



Zeiss's Camera Lucida with Two Prisms. — This camera lucida * 

 (Figs. 179 and 180) consists of a rectangular and an equilateral prism, 

 so combined in the mounting that the image of the pencil and 

 drawing paper is totally reflected twice, and is thus viewed erect 

 by the eye looking over the edge of the equilateral prism at the 

 image produced by the eye-piece of the Microscope. 



P (Fi«. 180) shows the direction of the ray from the drawing- 

 board ; after two reflections this ray emerges parallel to the optic 

 axis of the Microscope. As the angle between the two reflecting 



Fig. 179. Fig. 180. 



A. 



\ 



^5 



R2 



surfaces of the prism = 12°, the direction P must always be inclined 

 24° to the axis of the Microscope. If, in drawing with the prisms, 

 the plane of the paper were horizontal and the Microscope in a 

 vertical position, the drawing would be somewhat distorted, the 

 circular field being projected as an elliijse whose axis would be as 

 10 : 11. By inclining either the Microscope or the plane of the 

 drawing about 12° the amount of distortion is hardly perceptible. 

 To obtain a strictly similar projection the inclination should be 24°. 



The inclination of the upper surface of the equilateral prism 

 (which is supposed = 12° in the diagram) may be varied within 

 certain limits. An angle of 10° to 15° is necessary in order to have 

 total reflection in this prism for all directions within the range of the 

 microscope-field. 



The following description of the use of the apparatus is furnished 

 by M. Zeiss : — 



" To draw with the prism remove the eye-piece (after exact 

 adjustment of the object) and slide the ring, found under the prism, 

 over the tube of the Microscope, then put the eye-piece again in place. 



* This is sometimes known as the "Vertical Camera Lucida," as it is uaed 

 with the Microtcope in an upright position. 



