326 



ACCESSOKY APPARATUS 



and note the effects of opening and closing the slit by rotating the 

 screw, C (fig. 269) ; the lines can only be well seen when the slit is 

 reduced to a narrow opening. The screw H diminishes the length 

 of the slit, and causes the spectrum to be seen as a broad or a narrow 

 ribbon. The screw E (or in some patterns two small sliding knobs) 



FIG. 272. Upper half, map of solar spectrum, showing Frauiihofer lines. Lower 

 half, absorption spectrum, showing position of bands in relation to lines. 



regulates the quantity of light admitted through the square aperture 

 seen between the points of the springs, D D. Water tinged with 

 port wine, madder, and blood are good fluids with which to com- 

 mence this study of absorption bands. 1 As each colour varies in 

 refrangibility, the focus must be adjusted by the screw B, fig. 269, 



according to the part of the 

 spectrum that is examined. 

 When it is desired to see the 

 spectrum of an exceedingly 

 minute object, or of a small 

 portion only of a larger one, 

 the prisms are to be re- 

 moved by withdrawing the 

 tube containing them ; the 

 slit should then be opened 

 wide, and the object, or part 

 of it, brought into the centre 

 of the field ; the vertical and 

 horizontal slits can then be 

 partly shut so as to enclose 

 it ; and if the prisms are 

 then replaced and a suitable 

 objective employed, the re- 

 quired spectrum will be seen, 

 unaffected by adjacent ob- 

 j ects. For ordinary observa- 

 tions objectives of from two 



inches to ^-inch focus will be found most suitable ; but for very 

 minute quantities of material a higher power must be employed. 

 Even a single red blood-corpuscle may be made to show the 



1 A series of specimens, in small tubes, for the study of absorption-spectra, is 

 kept on sale by Mr. Browning : and the directions given in his How to work with 

 the Micro-spectroscope should be carefully attended to. 



273. 



