168 METHODS OF MICROSCOPICAL RESEARCH. 



disc is now manifest, proportion is affected by dis- 

 tance from the eye-piece, and with this gauge and a 

 stage micrometer, a drawing may be kept within 

 measurable bounds ; difficulties have existed as to 

 amplifications expanded by the camera-lucida; ab- 

 solute accuracy may be ascertained by the use of 

 micrometers as far as the eye-piece is concerned, 

 but the known diameter of a circle on which the 

 image is projected is an easy factor in such cal- 

 culations, beyond this, the circle is mechanically 

 useful, as, if the block should slip when using the 

 camera-lucida, there is an ascertained line for re- 

 adjustment. The light on the paper should be in 

 excess of that from the object ; speed and precision 

 are essential, quickly make recognised points and 

 lines, never attempt to draw detail, nothing fatigues 

 the eye or distracts the mind more than the pro- 

 longed employment of the camera-lucida. No ad- 

 advance can be made by its continued use, any 

 attempt at elaborate work ends in confusion. Cul- 

 tivate the " knack "of seeing at the same time, and 

 in the same position, the reflections and the image 

 of the tracing point (the hardest pencil, sharply 

 cut) ; it is not necessary to strain the sight to keep 

 the entire field always in view, there is a condition 

 of steady gazing, the eye not too close to the prism, 

 when PARTS of the object can be taken separately, 

 but this is a result of the facile use of the instru- 

 ment. The neutral tint glass or any form of reflec- 

 tor, giving only one, and consequently, a reversed 

 image, is useless for afterwards continuing a draw- 

 ing from direct vision, but with any description of 

 camera-lucida, the pencil, once placed on the paper, 

 should not, if possible, be lifted until all determining 



