ON THE THEOKT OF THE MICROSCOPE. 235 



detail readies 100 parts to the milimetre (that is when their 

 interspace is only j^^^^ inch) nothing remains visible except a 

 homogenous surface whatever magnifying power be used, or 

 whatever mode of illuminating (direct or oblique.) Even a couple 

 of lines ruled on a glass will, under the circumstances above stated, 

 be not otherwise distinguishable than as one broader line with 

 sharp outlines. With the most powerful immersion lens nothing 

 at all can be seen of the markings of Pleurosigma angm., and 

 even the coarse lines of Hipparchia Janeira remain unrecognisable 

 with a power of 200. In the case of granular objects and other 

 irregularly shaped particles, diffracted light cannot be completely 

 separated from undiffracted light, and accordingly there is no 

 absolute disappearance of all the particles ; but such indefiniteness 

 of image ensues that the finer particles of the preparation fuse 

 into a homogenous gray cloud. 



(ii.) When all light is shut off, excepting a single pencil of 

 diffracted rays, a positive image of the particles in the object 

 which caused the diffraction is formed, and appears more or less 

 brightly on the dark field, but without any detail. Ealed lines 

 appear as uniformly clear flat stripes on a dark ground. 



(iii.) Eut when not less than two separate pencils are admitted 

 the image always shews sharply defined detail, whether it 

 appears in th3 form of system of lines, (one or more sets) or of 

 separate fields; nor does it matter whether undiffracted light 

 passes in with the incident cones or not : that is to say, whether 

 the image appears on a bright or a dark field- If fresh pencils 

 be set in operation fresh details appear, but always different, 

 according to the degree of minuteness, or to the nature of the 

 markings ; and this detail is not necessarily cjnformalle either with 

 that of the image as seen hy ordinary illumination, or with the real 

 structure of the object as hnoivn or ascertained in other ways. In 

 respect to this last point the following particulars are noteworthy. 



(iv.) A simple series of lines will be always imaged as such 

 when two or more illuminating pencils are set in operation, but 

 the lines will appear doubly or trebly fine when, instead of the 



