ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 865 



course, quite untenable. I wish to show how these differences of 

 foci may be accounted for by the spherical aberration of the objective 

 used. 



Let fig. 180 represent the back of a 1/4 in. objective of 0*71 N.A., 

 focused on a Navicula cuspidata, illuminated by two oblique beams, 

 a, b, at right angles to each other ; a' will represent the diffraction 

 spectrum of the first order originated by the illuminating beam a, 

 and the fine longitudinal striae ; and 6' that by the beam b and the 

 coarse transverse strias. 



It is evident that if the lens has not its spherical aberration 

 properly balanced, the rays a a' passing through the outer zone of 

 the objective will have a shorter focus than b b'. In other words, 

 when the transverse striae are in focus you will have to focus the lens 

 further down before the longitudinal striaa appear. This is precisely 

 what occurs in practice. The experiment is nothing more nor less 

 than a refined kind of Abbe's test. 



For my own part, I prefer to test an objective by flooding it with 

 light from a large axial illuminating cone." 



Actinic Contrast in Photo-micrography.* — Dr. G. A. Piersol con- 

 siders that successful photo-micrography depends especially upon 

 three conditions : (a) having all parts of the object accurately in the 

 same plane ; (b) having a well-marked differentiation between the 

 elements of the tissues ; and (c) having the object so stained and 

 illuminated as to insure sufficient actinic contrast between it and the 

 surrounding field or background. 



The successful acquisition of the condition of actinic contrast, is 

 not always readily had. While the blue stainings (haematoxylin, 

 methyl-blue) are, of course, more actinically powerful than the reds 

 and browns, yet so much depends upon the individual specimen in 

 regard to opacity and thickness, that each case must be determined 

 for itself. While a thick section stained in carmine will yield but a 

 dark mass without detail, a similar section stained in hematoxylin 

 may furnish a satisfactory picture. But the days of thick sections 

 are past ; the question now is, how shall we stain and illuminate 

 the thinnest possible sections so as to yield good photographs ? 



While a very delicate section well stained with haematoxylin is 

 all that can be desired for examination, we shall soon find that 

 actinically it is far too transparent to produce a vigorous photograph, 

 there being insufficient actinic contrast between the general blue 

 colour of the field illuminated by the blue monochromatic light from the 

 ammonia-sulphate of copper cell, and the bluish purple of the section. 

 When the preparation of the specimen is under control, it will be 

 found advantageous to prepare a few sections as already suggested,! 

 by which the thinnest sections in the brown colours always markedly 

 impress the plate. 



In many cases, however, it is inexpedient to specially prepare 

 objects for photography. For such cases a very valuable adjunct will 



* Anier. Mon. Micr. Journ., vii. (1886) pp. 121-3. 

 t See this Journal, v. (1885) p. 559. 

 Scr. 2.— Vol. VI. 3 L 



