MICROSCOPIC ACCESSORIES. 47 



rangement of the prisms and lens, together with a microm- 

 eter for measuring the position of lines or absorption 

 bands. To use the microspectroscope, remove the tube 

 A, with the prisms, and insert the tube G in the place of 

 the eye-piece of the microscope. With the lowest power 

 object-glass which is suitable, and the slit opened wide by 

 the screw H, the object on the stage of the microscope, 

 illuminated by the mirror or condenser, is brought to a 

 focus, the tube A replaced and adjusted for focus by the 

 screw B, while the slit is regulated by c and H until a well- 

 defined spectrum is seen. To determine the position of 

 the absorption lines, remove the upper cover of the tube 

 A and replace it with that carrying the micrometer repre- 

 sented in Fig. 26. The mirror illuminates a transparent 

 line or cross, whose image is refracted by a lens c, mov- 

 able by a screw B, and reflected at an angle of 45 from 

 the upper surface of the prisms, so as to be seen upon the 

 spectrum. By means of the micrometer screw M, this is 

 made to move across the spectrum, so that the distance 

 between the lines may be determined. In order to com- 

 part the results given by different instruments, the 

 observer should measure the position of the principal 

 Fraunhofer lines in bright daylight, and mark them on 

 a cardboard scale, which may be preserved for reference. 

 By comparing the micrometric measurement of lines in 

 the spectrum of any substance observed by artificial light 

 with such a scale, their position may readily be seen. 



In using the microspectroscope some objects require a 

 diaphragm of small size, and others, especially with the 

 1J or 2-inch objective, a cap with a hole j'gth of an inch 

 in diameter over the end of the microscope, to prevent 

 extraneous light from passing through the tube. 



Nose-piece. For the purpose of facilitating observations 

 with objectives of different powers a revolving nose-piece 

 has been contrived, carrying two, three, or four objectives, 



