158 MANAGEMENT OF THE MICROSCOPE. 



when it is precisely in focus. The regulation of the amount of 

 light transmitted through the object is often of the very first 

 importance ; and no means of accomplishing this is so convenient 

 as a Graduating Diaphragm (§ 78). For some objects of great 

 transparence, the White-Cloud illumination (§ 87) may be had 

 recourse to with advantage. For the most difficult class of objects, 

 however, when viewed by lamp-light under the highest powers, it 

 is better to dispense with the Mirror altogether, placing the lamp 

 in the axis of the Microscope, so that its light shall fall directly on 

 the Condenser. 



11 9. There are many Transparent Objects, however, whose peculiar 

 features can only be distinctly made out when they are viewed by 

 light transmitted through them obliquely instead of axially ; and 

 this is especially the case with such as have their surfaces marked 

 by very delicate and closely-approximated furrows, the direction of 

 the oblique rays being then a matter of primary importance. Thus 

 suppose that an object be marked by longitudinal stria? too delicate 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 it in the directions A and B tending 



to obscure the stria? rather than to disclose 

 A them. But, moreover, if the stria? should be 



due to furrows or prominences which have 

 one side inclined and the other side abrupt, 

 D 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 sha- 

 B dow : 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 longitu- 

 dinal stria? to be crossed by others ; and these transverse stria? 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 illuminating pencil or the object must be moved a 

 quarter round. The simplest mode of attaining this end is to 

 make the Mirror capable of being turned into such a position, as to 

 reflect light into the object from one side and at a very oblique 

 angle ; and to give the Stage a rotatory movement, so that the object 

 may be presented to that light under every aspect. This is more 

 effectually accomplished in Nachefs Student's Microscope (Fig. 37), 

 than in any English instrument of equally simple construction which 

 the Author has seen. But where sufficient obliquity cannot be 

 given to the Mirror, nearly the same effect may be produced by 

 placing the Lamp in the desired direction, and interposing an ordi- 

 nary Condensing lens between it and the object. 



120. For objects of the greatest difficulty, however, it is better to 



