Accessory Apparatus 113 



draw-tube, then reinsert the eyepiece and set the neutral- 

 tint glass in position. The tinted glass is usually mounted 

 on an arm which has a joint, so that it may be turned out 

 of the way when not required without detaching the piece 

 of apparatus from the microscope. The distance of 10 

 inches between the eyepiece and the table is maintained, 

 whether the microscope has a 6 or a 10 inch tube-length. 



The Beale's neutral-tint possesses the disadvantage of 

 reversing the image that is seen with it. Mr. Ashe, there- 

 fore, devised a modification known by his name, which 

 overcomes this defect, while maintaining the simple 



FIG. 44. ABBE CAMERA LUCIDA. 



principle of the Beale's pattern. It is described in the 

 Journal of the Quekett Club. Of more expensive descrip- 

 tion, but considered the best at present made, is the Abbe 

 camera lucida (Fig. 44). The microscope may be used 

 in a vertical or any inclined position with this apparatus. 

 Its construction and the manner of using is as follows : 

 Mounted in a cap, which is fitted immediately above the 

 eyepiece, are two right-angle prisms ; these are cemented 

 together and form a cube. One of the cemented surfaces 

 is silvered, but a small central disc is left clear, through 



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