HISTOLOGICAL SUBJECTS. 209 



is reached. Slide the bellows back and forth until the image is 

 vaguely seen upon the ground glass, and then use the fine adjust- 

 ment until the image is perfect. It may be necessary to throw a 

 focussing cloth over the head and camera, in order to see the fine 

 details clearly. 



Sometimes one or more bright spots appear in the field, inter- 

 fering greatly with the definition. These are due to reflections 

 within the objective, or in some other part. By taking away the 

 ground glass and looking in with eyes almost shut, so as not to be 

 dazzled, one may generally detect where the reflections occur. 

 Sometimes the sleeve is not light-proof, or has become detached 

 and admits stray beams of light, which make confusion. When 

 the eye-piece is used, the objective may be focussed as usual, and 

 the stand bent horizontally. Then slip the sleeve over the end of 

 the tube, being careful not to disturb the adjustment ; and focus 

 the image upon the ground glass by moving the bellows. 



A more trying difficulty is curvature of the field. This 

 increases with the higher powers, so that often, out of a field five 

 inches in diameter upon the ground glass, not more than half-an- 

 inch can be brought into focus at one time. 



The best general effect is obtained by selecting some point to 

 focus upon, midway between the centre and the periphery. This 

 gives a field with moderately good definition throughout, and no 

 great contrasts between well-defined and blurred lines. 



If there is any special detail to be demonstrated, that of 

 course must be jjlaced in the centre of the field and focussed 

 without regard to the rest. As objectives are now constructed, the 

 only remedy that I am aware of is to make use of an eye-piece or 

 amplifier suited to the particular objective used. 



Lack of penetration may be somewhat obviated by a diaphragm, 

 as I have said before. The difference between the chemical and 

 visual foci is ascertained by interposing between the stage and the 

 condenser a deep violet-coloured solution (about 8 per cent.) of 

 pure cupric oxide in ammonia. If the cell is made of glass strips, 

 this fluid, which is exceedingly corrosive, soon acts upon the 

 cement and destroys it. When not in use, it is advisable to pour 

 the solution out, unless it is contained in a brown glass cell, such 

 as is used for holding fluid in spectrum analysis. 



Vol. VI. P 



