138 THE USE OF THE MICROSCOPE 



aberration on nearly parallel rays. (Dallingcr used the 

 reflecting prism with good effect.) With ordinary care 

 as to centering with the lamp, a 25-millimeter prism can 

 be used without reducing the condenser aperture, but a 

 30-millimeter prism is doubtless more error proof. Such 

 a prism can be used when the incident light is at right 

 angles to the first surface, and also when it falls more 

 obliquely still to the axis of the microscope; but, with 

 lessened obliquity, the reflection is total only for a range of 

 a few degrees. In actual use, with the source 25 centi- 

 meters away and about 20 centimeters high, and the instru- 

 ment inclined at about 35 degrees, the reflection may be 

 always total. If it is not total with the microscope vertical, 

 the lamp can be lowered. When a vertical microscope, 

 however, is regularly used, the reflecting side of the prism 

 should be silvered. The writer has used a well-made 

 25-millimeter prism instead of a mirror for five years of 

 daily work, with satisfaction. Hence, the plane mirror 

 need not be retained on the best research microscopes, but 

 may well be replaced by a prism. 



With a small diaphragm on the source of light, the center- 

 ing of this spot of light by moving the mirror or prism 

 requires to be done repeatedly. If the lateral and central 

 pivots moved in sleeves, more ease and greater accuracy 

 would be attained in this adjustment. 



When a good condenser is used, there is no need in ordi- 

 nary work to swing the mirror or prism bodily from side to 

 side; nor is there need to shift the mirror or prism up or 

 down. It should be fixed centrally on the tailpiece. In 

 the absence of a concave mirror on the reverse side, the 

 lack of centering of the prism or plane mirror, described 

 by Coles (46) and usually found, should no longer exist; and 

 the plane of the mirror or the prism base can be properly 

 centered with the optic axis. The centering of the ordinary 

 plane mirror, as now fitted, is usually improved by shorten- 

 ing its stem 3 or more millimeters. 



The Stage. — By use of an ordinary sliding bar on a 

 square stage, in the absence of a mechanical stage or object 



