MR. SORBY'S CELLS. 251 



of the milled head H. Focus the object and then gradu- 

 ally close the slit till a good spectrum is obtained, and 

 this may be further improved by throwing the object a 

 little out of focus. Each portion of the spectrum differs a 

 little from adjacent parts in refrangibility, and delicate 

 bands or lines can only be brought out by focussing their 

 own parts of the spectrum, which may be done by means 

 of the milled head B. 



One advantage which the micro-spectroscope possesses 

 is that extremely small quantities may serve for observa- 

 tion, and for this purpose powers varying from ^-inch to 

 ^o-inch may be used. 



It is very easy to have the tints of the solutions ex- 

 amined too intense, so that nothing but a general and 

 indefinite absorption appears throughout the entire spec- 

 trum. Nothing, perhaps, can show this better than a 

 solution of permanganate of potash or Condy's pink 

 fluid. 



Mr. Sorby has devised what he has termed a wedge cell, 

 in which to view the liquid for its absorption ; it is shown 

 in Fig. 217. By this means an increasing thickness of fluid 

 and consequent intensity of colour can be brought under 

 observation with the minimum of trouble. Again, very 



FIG- 217. FIG. 218. 



minute quantities of solutions may be examined in the 

 slide shown in Fig. 2 1 8, made of a piece of barometer tube 

 cut and ground evenly, and then cemented to a piece of 

 ordinary glass. 



Mineral sections, crystals, blowpipe-beads, &c., should 



