Scales of Insects and Fish. 47 



accompaniment of the large and expensive ones. This 

 apparatus consists of two prisms, one arranged to 

 fasten below the stage, the other to slip over the 

 eye-glass ; and the object to be shown by transmitted 

 light from the mirror. By this arrangement, various 

 splendid colours are exhibited, and the effect is further 

 heightened by placing on the stage immediately under 

 the object, a slide containing a thin plate of the 

 mineral " selenite," which, when mounted in balsam, 

 is perfectly transparent, indeed invisible, in the 

 ordinary mode of illumination. A description of the 

 appearance of the eel's scale, when viewed with the 

 polarizing apparatus, will serve as a specimen of the 

 effects produced by this mode of exhibition. 



The scale (fig. 9), which can with difficulty be seen 

 by common light, now appears, in a particular arrange- 

 ment of the prisms, to be projected on a black ground, 

 and to be of a beautiful silvery grey, nearly white at 

 the edges, and softly shaded to a deeper tint at the 

 centre ; and conspicuously placed over its surface are 

 two large black marks, similar to two V's placed 

 point to point, thus >~<^ making a figure like a St. 

 Andrew's cross over the whole scale ; and these also 

 are very softly shaded at their edges. Now I make 

 the upper prism revolve slightly ; the two V's com- 

 mence to revolve also, but presently become much 

 wider, and gradually change to a pale brown, while 

 the vacant part of the slide changes from black to 

 light grey ; till, on the prism having turned one quarter 

 of the way round, two pale and nearly white marks 

 occupy the places of the original black V's, the broad 



