ENGLISH AND AMERICAN MODIFICATIONS 



285 



When this form of camera is used, the paper upon which the 

 object is received should be tilted to the same plane as the stage of 

 microscope upon which the object rests, as this will pi-event any 

 marginal distortion. 



Another extremely good and easily applied 

 modification of the Abbe form is manufactured by 

 Bausch and Lomb. and is illustrated in fig. 225. 

 The Abbe prism is used as in the large Abbe 

 drawing camera ; the mirror is reduced in size 

 and is fixed. The path of the light is seen to be 

 the same as the white dotted lines and arrows 

 show, as in the complete form of Abbe ; and the 

 camera may be swung back when not in use, as 

 shown in the dotted outline. We can testify that 

 the image off both object and pencil-point are clear, 

 and this instrument can be used with most eye-pieces ; but cannot for 

 complete results be counted equal to the drawing camera of Abbe. 



The Editor has used with great facility and success a camera devised 

 by Dr. Hugo Schroder, and produced by Messrs. Ross. It is figured 

 at 226, and consists of a combination of a right-angled prism (fig. 

 227) ABC, and a rhomboidal prism D E F G, so arranged that when 



FIG. 224. Swift's 

 camera lucida on 

 the Abbe principle. 



FIG. 225. Bausch and Lomb's modification of Abbe's camera. 



adjusted very nearly in contact (i.e. separated by only a thin stra- 

 tum of aii-) the faces B C and D E are parallel, and consequently 

 between D E and B E' they act together as a thick parallel plate of 

 glass through which the drawing paper and pencil can be seen. 

 The rhomboidal prism is so constructed that when the face G F is 

 applied at right angles to the optic axis of the microscope, the axial 

 ray H passes without refraction to I on the internal face E F ; 

 whence it is totally reflected to J in the face D G . At J a part of 



