158 WM. WEDB ON " NORERT's TESTS." 



upon, or too forcibly call attention to, the different appearances 

 of specimens Nos. 1 and 2, as compared with No, 8, upon 

 shifting the focus. With high powers the plane of observation 

 does not include the whole of the depth of the incisions at the 

 same moment — in other words it does not include at the same 

 moment the surface of the glass and the lowest part of the 

 incisions at any one stage of the focussing. The higher the power 

 the less the depth of the plane of observation, or, as is well under- 

 stood by the expression, the less the penetration. This likewise is 

 easily proved by focussing downwards, and finding the first appear- 

 ance is that of the upper surface of the glass, with clearly defined 

 holes, which would seem to be continued through the substance ; 

 but, upon focussing a little lower, the upper surface is entirely lost 

 to view, and the apparent holes through the glass become greyish 

 black lines. Whence come these coloured lines ? The glass is 

 comparatively white ! Why do not all the incisions present this 

 dark appearance at the same moment? 



At some phases of the focussing of the fine bands one incision 

 will present two black lines. Whence come they ? It is necessary 

 lo understand something of this jDhenomena before proceeding 

 further with the subject. Microscopists are well aware of the 

 polariscopic effects of colour, and of the fact of ihose^olours being 

 produced by refraction, or the bending of the rays of light at 

 particular angles so as to produce only a portion of Frauhenhoffer's 

 lines — the colour depending upon the particular angle of refraction 

 and the particular portion of the solar spectrum brought under 

 observation. To this polarization of the light by the bending of 

 the rays transmitted through one bevelled side of the incision, in- 

 tersecting, commingling with, and crossing the opposite rays, bent 

 in an opposite direction through the other bevelled side of the 

 incision is, very clearly, to my mind, to be attributed these em- 

 barrassing black lines. Upon examining specimen No. 9 with 

 the unaided eye, and by powerfully reflected light at a particular 

 angle, the whole of the solar spectrum is brilliantly exhibited, but 

 that is due to the combination of reflection and refraction, while if 

 it be possible to absolutely cut off the top light, and to absolutely 

 destroy the reflection of the top light from Nobert's Test, the 

 polarised refraction of the transmitted light would still be present 

 in the black lines. The phenomena of these black lines become 

 more involved by the fact of the different lengths of the sides of 



