THE SPECTRUM MICROSCOPE. 69 



of the absorption-bands. If they are dark, narrow, well- 

 defined, and lie close together, as in the case of partially 

 opaque crystalline blow-pipe beads of borax containing 

 deposited crystals of oxide of lanthanum with oxide of 

 didymium, a somewhat powerful dispersion is not only 

 admissible, but quite necessary to separate some of the bands. 

 If, however, they are broad and faint like those seen in the 

 spectra of many of the colouring matters found in animals 

 and plants, a powerful dispersion spreads them over such a 

 wide space, and makes the shading off so gradual, that the 

 eye can scarcely appreciate the extra amount of absorption ; 

 whereas, when a lower dispersive power is used, a well- 

 marked absorption-band can easily be seen." 



" There can be no doubt that it is a great advantage to 

 have a number of prisms of different dispersive power, so that 

 in all cases the most suitable may be used; but at the same 

 time some observers might not wish to have more than one, 

 and thus it becomes important to decide what amount of 

 dispersion is the best for the generality of objects — is suffi- 

 ciently great to divide narrow, closely-placed bands, and yet 

 not so great as to prevent our seeing broad and fainter. No 

 magnifying power whatever is applied to thd spectrum itself 

 in the instrument now under consideration." 



Mr. Sorby then goes on, " As described in some of my 

 former papers,^ the compound, direct-vision prisms first made 

 for me by Mr. Browning Avere composed of two rectangular 

 prisms of not very dense flint-glass, and three of crown glass, 

 one being rectangular, and the other of an angle of about 75°. 

 This combination gives a dispersive power which shows faint 

 bands very well, but is not enough to divide the narrow and 

 close bands seen in the spectra of a few substances. Mr. 

 Browning then made prisms of similar construction, only that 

 very dense flint glass was employed. This combination gives 

 about double the former dispersion, which divides narrow 

 and close bands admirably, but sometimes shows broad and 

 fainter bands so very badly that they can scarcely be recog- 

 nised. It thus appeared to mc that, if only one compound 

 prism be supplied with the instrument, the best dispersive 

 power would be intermediate between these two extremes. 

 At the same time much would depend on the particular 

 purpose to which the instrument Avas applied, and also, to 

 some extent, on the individual differences between different 

 observers. 



^ ' Popular Science Review, vol. v., 186G, pp. 06 —71 ; ' Brit. As. lleport,' 

 1865 (pt. 2), p. 11. 



