436 Mr. H. C. Sorby on Analysis of Animal and Vegetable [Apr. 11, 

 numbered the bands, and given below the principal Fraunhofer lines. 



012345 6 7 8 9 10 11 12 



ABCD E6F G 



The centre of the bands is black, and they are shaded off gradually at each 

 side, so that the shaded part is about equal to the intermediate bright 

 spaces. Taking, then, the centres of the black bands as 1, 2, 3, &;c., the 

 centres of the bright spaces are H, 2|, 3|, &c., the lower edges of each |, 

 1|, &c., and the upper 1|, 2\, &c. We can easily divide these quarters 

 into eighths by the eye ; and this is as near as is required in the subject 

 before us, and corresponds as nearly as possible to part of the whole 

 spectrum, visible under ordinary circumstances by gaslight and dayhght. 

 Absorption-bands at the red end are best seen by lamplight, and those at 

 the blue end by daylight. 



On this scale the position of some of the principal lines of the solar 

 spectrum is about as follows : — 



A .... I B .. .. 1| C ....2| D .. 3i 

 E....5fi 5....6y3_ F...,7i G....10f 



At first I used plates of selenite, which are easily prepared, because they 

 can be split to nearly the requisite thickness with parallel faces ; but I found 

 that its depolarizing power varies so much with the temperature, that even 

 the ordinary atmospheric changes alter the position of the bands. Quartz 

 cut parallel to the principal axis of the crystal is so slightly affected in this 

 manner, as not to be open to this objection, but is prepared with far greater 

 difficulty. The sides should be perfectly parallel, and the thickness about 

 •043 inch, and gradually polished down with rouge until the sodium4ine 

 is seen in its proper place. This must be done carefully, since a difference 

 of inch in thickness would make it decidedly incorrect. I have 



prepared such plates, corresponding to my own, and placed them in the 

 hands of Mr. Browning and of Messrs. Becks, so that any one wishing to 

 adopt a similar scale may be able to do so more accurately. 



The two Nicol's prisms and the intervening plate are mounted in a tube 

 and attached to a piece of brass in such a manner that the centre of the 

 aperture exactly corresponds to the centre of any of the cells used in the 

 experiments, which are all made to correspond in such a manner that any 

 of them, or this apparatus, may be placed on the stage and be in the proper 

 place without further adjustment, which, of course, saves much time and 

 trouble. 



4. Symbols used to describe Spectra. 

 In order to describe spectra in my note-book or in print, I have devised 

 a simple notation, employing types in constant use. Instead of writing an 



