WITH THE MICROSCOPE. 265 



the lens not have been carefully corrected by a back lens, for the low 

 powers, as previously advised, proceed to make the necessary allow- 

 ance, which experience has determined, by turning back the screw of 

 the fine motion, the number of divisions or parts required as marked 

 on the milled head. If not known, commence the experiment as 

 before stated, p. 255, and note the particulars. A card covered with 

 black cloth or velvet, with its lower edge turned at right angles and 

 deeply notched, is now rested on the stem of the microscope against 

 the end of the achromatic condenser, facing the prism, and this latter 

 protected by a thick fold of chamois leather from the sun's rays. 

 Care must be taken that neither surface of the prism is soiled by 

 vapour or finger marks ; nor must the concentrated sunlight be per- 

 mitted to remain longer on the object than is actually required in 

 focussing, or it may become uncemented, and if not injured, it may 

 slip completely out of the field. 



This apparatus, if used in the open air, could have the micro- 

 scope end to move instead of the camera, but this method is very 

 inconvenient, when used near an open window, from the difficulty at 

 times to place the prism outside the plane of the window or in its 

 best position. 



If the higher powers be used, needing the screw adjustment for 

 the correction of the error introduced by the thin glass cover, we 

 find it best to make this as nearly as we can when examining the 

 object in the microscope, then testing, with the collar set to that 

 figure, the image on the ground-glass screen. If the image here 

 seems moderately sharp, under the best focussing, a trial is made by 

 shifting the collar a very little and watching the appearance of the 

 image ; sometimes a very trivial alteration will bring out fine mark- 

 ings much more distinctly ; the focus, if this be the case, will also 

 often require readjustment ; but before making this, it will perhaps, 

 if only trifling, be as well to test a plate, when, should the negative 

 be found defective in the parts most sharply focussed, try another, 

 withdrawing the objective by the milled-headed screw. It is often 

 in this way that the qualities of an objective are rendered evident. 

 It sometimes becomes very troublesome to ascertain these points 

 for a variety of objects and covers. Assuming that the plane of 

 the greyed glass screen, and that occupied by the sensitised plate, 

 are strictly alike, if the second image be out of focus, test again with 

 the apparent necessary change learnt from a close examination of 

 the negative, and the image on the screen. When once correctly 

 found, note the division of the screw collar and the distance in inches 

 at which the camera stands fixed by the pegs, and seen by the figures 

 on the guides, as necessary for that objective used at the same 



