VARIOUS FORMS. 



as shown in fig. 3, the ray A B from the object entering the face 

 of the prism perpendicularly, and being reflected at B to the eye 

 ate. 



If two reflections be used, a quadrangular prism, having two 



Fig. 3. Fig. 4. 



angles of 67^, one right angle, and one of 135, is applied, as 

 shown in fig. 4. The course of the ray from the object to the 

 eye being A B c D. 



In the preceding cases, we have supposed the observer to see 

 the object by reflection, and the paper and pencil directly ; but 

 it is evident that the conditions may as easily be reversed, so that 

 the object may be seen directly, and the paper and the pencil by 

 reflection. Thus we may suppose the plane mirror M M' in fig. 1, 

 to be silvered in the upper instead of the lower surface, and the 

 observer looking from E horizontally to see the object directly 

 through the unsilvered part, while he sees the paper and pencil 

 by the reflection from the silvered part. 



This method is in many cases found more convenient than that 

 first described. 



7. In some forms of the instrument, the observer looks at the 

 object through a small hole made in a plane reflector, placed at an 

 angle of 45 in the direction of the paper ; the diameter of the 

 hole being less than that of the pupil. In this case, while the 

 object is seen directly through the hole, the paper and pencil 

 are seen by reflection from the surface of the reflector surrounding 

 the hole ; this is the form of the camera-lucida applied to the 

 microscope by Professor Amici. 



8. Whatever be the form of the camera, the visual magnitude 

 of the image projected on the paper as seen by the eye applied 

 to the instrument, is the same as the visual magnitude of the 

 object seen directly, and this will be the case at whatever distance 

 from the camera the paper may be placed. It follows from this, 

 that the actual magnitude of the picture projected on the paper 

 will be greater or less according to the distance of the paper from 

 the camera, and that consequently the observer, by regulating the 



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