738 THE MICROSCOPE. 



be best understood from the following figure, in which 

 the bands are numbered, and given below the principal 

 Fraunhofer lines. The centre of the bands is black, and 



0123456 78 9 10 11 12 



< R ^> IIHIiillHH 



1 1 1 1 i i i 



Fig. 355. A BCD E b F o 



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 spaces are 1J, 2J, 3|, &c., 

 the lower edges of each J, If, &c., and the upper 1J, 2J, 

 &c., we can easily divide these quarters into eighths by 

 the eye ; and this is as near as is required in the sub- 

 ject before us, and corresponds as nearly as possible to 

 1-1 00th part of the whole spectrum, visible under ordinary 

 circumstances by gaslight and daylight. 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 f B 1J C 2| D 3| 



E 5H 6 6^ F,.;...74 G....:10- 



At first plates of selenite, which are easily prepared, 

 were used, because they can be split to nearly the re- 

 quisite thickness with parallel faces ; but its depolarising 

 power varies so much with the temperature, that even 

 the ordinary atmospheric changes alter the position of the 

 bands. However, 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, the thickness about '043 inch, and gradually 

 polished down with rouge until the sodium- line is seen in 

 its proper place. This must be done carefully, since a 

 difference of l-10,000th inch in thickness would make it 

 decidedly incorrect. 



The two Nicol's prisms and the intervening plate are 

 mounted in a tube, and attached to a piece of brass in such 



