MEMBRANES BOUNDED BY ONE PLANE, ETC. 219 



raising the tube the central part will appear red, and the edge 

 bluish, whilst lowering it below the virtual focus will produce the 

 opposite arrangement of the colours, assuming in both cases that 

 the objective is under-corrected. Since this is generally the case 

 with Microscopes, minute pores, cracks, furrows, &c., will always 

 appear reddish, if we focus the instrument to about their centre, 

 and indeed more or less so according to the peculiarity of the 

 instrument. The bluish fringe, too, though of very slight breadth, 

 is in many cases perceptible. 



2. MEMBRANES BOUNDED BY ONE PLANE AND ONE UNDULATING 



SURFACE. 



The elevated portions of membranes of this kind (Fig. 119) 

 evidently act as convex lenses, and their depressions as concave 



lenses. The distribution of m n 



light and colour must there- 

 fore in general be as follows : _ |_? L_^li-.^3C| ^ 



with the highest focal ad- Hi/' - $A 



justment (in ri) to the real FlG * 119< 



images of the projections, the latter will appear bright, and the 

 depressions dark or reddish, according to the curvature; if we 

 lower the tube, the projections will have a bluish tint, and the 

 depressions will remain red, until we reach the plane (p q) of 

 their virtual images. With still lower focal adjustment, the latter 

 become bluish ; the reddish fringes, which they surround, then 

 take the place of the elevations blended in pairs which now 

 appear as the depressions did when the focal adjustment was 

 higher. What colours will alternate with a given adjustment 

 depends, of course, upon the curvature of the upper surface. 



If the elevations project very much, and rest upon a somewhat 

 narrow base, in addition to the phenomenon just mentioned 

 reflexion occurs, as observed in minute corpuscles, globules of 

 mercury, &c., which we shall more fully discuss hereafter. This 

 obtains, for instance, with many spiral and annular vessels, with 

 branch-like crystalline films (which act in the same way as a 

 thickening of the object-slide), the siliceous envelopes of diatoms, 

 &c. In these objects it frequently occurs that single fibres, even 

 if quite isolated, appear reddish on lowering the focal adjustment. 



