146 THE MICROSCOPE AND ITS REVELATIONS. 



case, indeed, in which it is desired to ascertain the effect of variety in 

 the method of illumination, the effects of alterations in the distance of 

 the condenser from the object should be tried; as it will often happen 

 that delicate markings become visible when the condenser is a little out 

 of focus, which cannot be distinguished 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 or Iris Diaphragm ( 98). For some 

 objects of great transparence, the White-Cloud illumination ( 109) may- 

 be had recourse to with advantage. 



145. There are many Transparent Objects, however, whose pecu- 

 liar 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 deli- 

 cate 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 striae too delicate to be seen by ordinary 

 direct light; the oblique light most fitted to bring them into view will be that 



A proceeding in either of the directions c or D; that which, 



falls upon it in the directions A and B tending to obscure 

 the striae rather than 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: 

 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 simplest mode of obtaining 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 (which is best done by the 

 Zentmayer arrangement) ; and to give the Stage a rotatory movement, so- 

 that the object may be presented to that light under every azimuth. 



146. For objects of greater difficulty, however, it is better to have 

 recourse to the Accessories already described ( 101-108), which are 

 specially provided to furnish oblique illumination in the most effectual 

 manner. A good example of the variety of appearances which the same 

 object may exhibit, when illuminated from different azimuths, and with 

 slight changes of focussing, is shown in Fig. 113, which represents por- 

 tions of a valve of Pleurosigma formosum as seen under a power of 1300 : 

 diameters; the markings shown at A, B, and c, being brought-out by oblique- 

 light in different directions, which, however, when carefully used, does 

 not produce these erroneous aspects; whilst at D, is shown the effect of 

 axial illumination with the Achromatic Condenser. It cannot be too 

 strongly impressed on the young Microscopist, however, that the special 

 value of very oblique illumination is limited to the resolution of ' test- 

 objects;' and that for the ordinary purposes of scientific study and 

 research, axial illumination is generally preferable. As in regard to the 

 qualities of Objectives ( 55), so in respect to Illumination, may 



