QUARTZ SCALE. 255 



ammonia (ferrous amm. sulphate), and examine again. The 

 two bands will gradually fade and be replaced by a fainter, 

 though broad and single, band, as shown in G. If this be 

 allowed to oxidise in the air, the two original bands of 

 haemoglobin can be seen again, and on adding a little citric 

 acid the spectrum becomes changed to that seen in H. 

 The citric acid has changed the haemoglobin into haematin, 

 and this latter can be further deoxidised by contact with 

 ferrous sulphate, when it produces the spectrum shown at I. 

 . Other spectroscopic changes can be induced in blood, but 

 for these we must refer the reader to the tenth Report of the 

 medical officer of the Privy Council, 1867. 



In all work such as this, it is advantageous to be able to 

 throw up a second spectrum alongside the first for com- 

 parison; and in all good instruments this is provided for 

 by fixing a small prism across half the slit, which receives 

 its light from the side of the tube, dispersing it upwards to 

 the eye. 



For registering the position of the absorption bands 

 two methods have been devised, (i) the quartz scale of 

 Mr. Sorby, and (2) the bright spot micrometer of Mr. 

 Browning. The first is constructed of two Nicol prisms, 

 with an interposed plate of quartz, about "043 of an inch 

 in thickness, cut parallel to the principal axis of the crystal. 

 This produces twelve interference lines or black bands in the 

 spectrum, and is adjusted so that the sodium line of the 

 solar spectrum is exactly midway between the third and 

 fourth band, reckoning from the red end. This adjustment 

 is performed by varying the thickness of the quartz plate, 

 by no means an easy operation, as a very slight difference 

 in thickness upsets the scale completely. 



The Sorby quartz scale is small and convenient, and is 

 shown at Fig. 220. 



The use of this scale monopolises one of the two spectra, 



