ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 339 



are plenty of such objectives in existence, though it is not unnatural 

 that the belief that the surplus rays passing through the marginal 

 zones are not image-forming rays, should lead the optician to con- 

 centrate his attention on the central corrections to the comparative 

 neglect of the marginal. Any experiments with such objectives will, 

 of course, be useless. 



The simplest experiment of all is to take a homogeneous-immersion 

 objective of large aperture, say 1'25, and put a stop (of cardboard 

 or tin-foil) on the back lens, leaving only a small clear annulus of the 

 extreme marginal rays. "With sufficient obliquity of the illumination 

 the image of the object will be seen perfectly delineated either on a 

 dark or bright field. 



Another experiment is to take a similar objective and focus it to 

 any rather coarse object with sharp outlines (say lines ruled in a thin 

 silver film on a cover-glass and balsam mounting) and illuminate by a 

 narrow pencil from an immersion condenser such as a hemisj^here, so 

 as to be able to get at pleasure a very oblique pencil. If the 

 illumination is changed from central to very oblique until the field 

 begins to darken, a well-defined image of the outlines will be seen with 

 slightly coloured borders only. If the illuminating apparatus admits 

 of a rapid change of the incidence without any alteration of the object, 

 it can be established that the image with the oblique beam is obtained 

 at the same focus of the eye-piece, and that no change of adjustment 

 is required for distinct vision. Looking down the tube when the 

 eye-piece is removed, the oblique pencil will be seen emerging close 

 to the margin of the clear aperture of the objective. Thus the 

 marginal zone transmits image-forming rays. 



Or focus the objective on a good specimen oi Amphipleura jpellu- 

 cida and use sufficiently oblique illumination for seeing the lines 

 clearly. On removing the eye-piece, placing the 

 pupil on the air-image of the diatom and look- 

 ing down on the lens, the direct incident beam 

 will be seen emerging as a bright spot and ex- 

 actly opposite and close to the margin a faint 

 bluish light (a portion of the first spectral beam) 

 — see Fig. 77. If now a small piece of paper is 

 placed on the back lens of the objective so as to 

 just cover up the blue light, and the eye-piece 

 is replaced, the diatom is still visible, but all the 

 striation which was imaged by the blue marginal light has entirely 

 disappeared. The latter must therefore consist of image-forming 

 rays. 



Other instances may be shown by using the vertical illuminator, 

 in the manner suggested by Mr. Stephenson,* in which case, when 

 transparent objects are used, the light within 82^ passes through, and 

 is not brought to the eye. The image is therefore seen by the 

 marginal rays. 



Indeed, the experiments which show the absurdity of the notion 



* See this Journal, ii. (1879) p. 266. 



