OBLIQUE ILLUMINATION. 173 



instrument, achromatic condensed light may be thrown 

 upon any object on the stage. The prism has the usual 

 swinging motion, accomplished by the frame turning on 

 two screws, one of which is seen at c, at the end of the 

 semicircle. 



A thin stage is of importance when a very oblique 

 pencil is used, and, to a certain extent, the larger its lower 

 aperture, the more oblique will be the rays we are enabled 

 to transmit to the object. For this reason the thin stage 

 introduced by such makers as Powell and Lealand, is in 

 every way an improvement on the thicker stage of the 

 older instruments. A revolving stage also possesses ad- 

 vantages : it enables the observer to keep the object in 

 the centre of the field of view, and while its various parts 

 are thus presented in succession to rays of greater or less 

 obliquity, an insight into the structure is often afforded 

 which could not otherwise have been obtained. " Thus, 

 suppose that an object be marked by longitudinal stria?, 

 too faint to be seen by ordinary direct light, the oblique 

 light most useful for bringing them into view will be that 

 proceeding in either of the directions c and D ; that which 

 falls upon it in the directions a and B 

 tending to obscure the striae rather than a 



to disclose them. But if the striae should 

 be due to furrows or prominences which, 

 have one side inclined and the other side 

 abrupt, they will not be brought into 

 view indifferently by light from c, or 

 from d, but will be shown best by that 

 which makes the strongest shadow ; B 



hence, if there be a projecting ridge, 

 with an abrupt side looking towards c, it will be best seen 

 by light from d ; whilst, if there be a furrow with a steep 

 bank on the side of c, it will be by light from that side that 

 it will be best displayed. But it is not at all unfrequent 

 for the longitudinal striae to be crossed by others ; and 

 these transverse striae will usually be best seen by the light 

 that is least favourable for the longitudinal, so that, in 

 order to bring them into distinct view, either the illumi- 

 nating pencil or the object must be moved a quarter 

 round." 



D 



