174 MANAGEMENT OF THE MICROSCOPE. 



of Amici's prism, however (which serves both as mirror and con- 

 denser, 60), will save the necessity of any other provision of 

 this kind. It is when objects are thus illuminated by oblique 

 light, and when their markings are of such a kind as to be best 

 or to be only shown by light falling upon them in one particular 

 direction, that we derive the greatest advantage from the power 

 of giving a rotatory movement either to the object or to the illu- 

 minating apparatus. Thus suppose that an object be 

 marked by longitudinal striae, too faint to be seen by 

 ordinary direct light ; the oblique light most fitted to 

 bring them into view, will be that proceeding in 

 either of the directions c or D ; that which falls upon 

 B it in the directions A and B, tending to obscure the 



striae rather than to disclose them. But, moreover, 

 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 ; 

 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 favorable for the longitudinal ; so that, in 

 order to bring them into distinct view, either the illuminating 

 pencil or the object must be moved a quarter round. The re- 

 volving action with which the stage of Mr. Ross's Microscope is 

 provided ( 37), enables this movement to be given to the object 

 without any displacement of its image, which, of course, exe- 

 cutes, to the eye of the observer, a rotation in the opposite di- 

 rection. In other microscopes, however, it is difficult to give a 

 rotation to the object, by causing the object-platform to turn 

 upon its axis, without throwing the object out of the field ( 38); 

 though this may be accomplished, by such an adjustment of the 

 traversing movement, as shall bring the centre of the tube on 

 which that platform turns round, into the visual axis of the 

 microscope or, if this adjustment cannot be conveniently made 

 in the first instance, by keeping the right hand constantly in 

 action upon the milled heads of the stage movement, whilst the 

 left hand rotates the object-platform, so as, by means of the 

 former, to correct the displacement of the object occasioned by 

 the latter. It may be sufficient, however, to examine the object 

 in several different positions, so that the appearances it presents 

 in each may be compared, without thus watching the transition 

 from one to the other. 



90. There are many kinds of transparent objects, especially 

 such as either consist of thin plates, disks, or spicules of siliceous 

 or calcareous matter, or contain such bodies, which are peculiarly 



