124 THE MICROSCOPE. 



screw at right angles to c, but which from its position 

 could not be shown in the cut, regulates the slit hori- 

 zontally. This screw has a larger head, and when once 

 recognised cannot be mistaken for the other. D D is an appa- 

 ratus for holding a small tube, that the spectrum given 

 by its contents may be compared with that from an object 

 on the stage. E is a square-headed screw, opening and shut- 

 ting a slit to admit the quantity of light required to form 

 the second spectrum. A light entering the round hole 

 near E, strikes against the right-angled prism, which we 

 have mentioned as being placed inside the apparatus, and 

 is reflected up through the slit belonging to the compound 

 prism. If any incandescent object be placed in a suitable 

 position with reference to the round hole, its spectrum 

 will be obtained, p shows the position of the field lens of 

 the eye-piece. G is a tube made to fit the microscope to 

 which the instrument is applied. To use this instrument 

 insert G, like an eye-piece in the microscope tube, taking 

 care that the slit at the top of the eye-piece is in the same 

 direction as the slit below the prism. Screw on to the 

 microscope the object-glass required, and place the object 

 whose spectrum is to be viewed on the stage. Illuminate 

 with the stage mirror if it be transparent; with mirror, 

 Lieberkiihn, and dark well, by side reflector, or bull's-eye 

 condenser if opaque. Remove A, and open the slit by 

 means of the milled-head, not shown in cut, but which is 

 at right angles to D D. When the slit is sufficiently open 

 the rest of the apparatus acts like an ordinary eye-piece, 

 and any object can be focussed in the usual way. Having 

 focussed the object, replace A, and gradually close the slit 

 till a good spectrum is obtained. The spectrum will be 

 much improved by throwing the object a little out of 

 focus. 



Every part of the spectrum differs a little from adjacent 

 parts in refrangibility, and delicate bands or lines can only 

 be brought out by accurately focussing that particular part 

 of the spectrum. This can be done by the milled head B. 

 Disappointment will occur in any attempt at delicate in- 

 vestigation if this direction be not carefully attended to. 



At E a small mirror is attached, which is omitted in the 

 diagram to prevent confusion. It is like the mirror belo^ 



