WORKING WITH LOW POWERS. 223 



possible, and the outlines of the band clear and distinct. 

 By this time the operator will have probably discovered 

 that a slight rotation of the main tube will separate 

 the horizontal band into two parts, or, as some of my 

 pupils express it, " two tongues." The best position is 

 when these are made to coalesce as completely as possible. 

 It is also probable that in the attempts thus far made 

 that the ima^e of the scale has been well seen. When 

 this occurs it should be at once focussed. The next 

 procedure is to correct the objective; the correction 

 obtained by transmitted light will not suffice for the 

 purpose in hand. It will be noticed that as the glass is 

 made to approach the correct adjustment, the horizontal 

 band of light will be correspondingly improved. So 

 true is this, that one might almost be governed thereby 

 in the adjustment of the objective, Having got thus 

 tar along, and without any serious mishap, it will be 

 easy, by closing the shutte'r, to admit the precise and 

 most favorable amount of light, and also to try the 

 effect of sundry very slight changes in the position of 

 the main tube, glass disk and lamp. Very beautiful 

 resolutions are sometimes obtained by bringing the lamp 

 within five or six inches and interposing the bull's eye 

 condenser, flat side to the lamp, in which case the shut- 

 ter must be further closed. It will happen also, occa- 

 sionally, that the best exhibition of striae on very diffi- 

 cult objects, such as extremely close fnistulia saxonicas, 

 is when the striae are placed at right angles to the hori- 

 zontal band of light. 



Now, should the manipulator meet with tolerable sue- 



