34 Transactions of the Society. 



but for full illumination the facet at 30° is required. Light 

 emerging from the slide at 30^ is, of course, bent down so as to 

 strike the under surface of the cover at about 49°, and in this light 

 the dry diatom may be splendidly resolved. In balsam, light at 

 about the same angle (49°) seems to resolve the diatom best. 

 With Am2)hi2jleura pellucida the light should in all cases strike 

 the diatom end on, or it will not be resolvable. The brilliancy of 

 the field also must be kept in due subordination to the influence of 

 the difiraction image, and as the following method of procedure 

 makes this very difiicult object quite easy, I may perhaps be per- 

 mitted to describe it. 



1. By means of a four-tenths objective, a diatom should be 

 selected, centred, and turned so as to lie exactly north and south 

 in the field. 



2. If light at 49° is needed, the corresponding facet of the 

 prism should be turned to the front. The Microscope tube should 

 be inclined through the complementaiy angle (41^), so that the 

 facet stands vertical. 



3. The lamp flame— edge on — should be set on a level with the 

 object, and at eight inches distance. 



4. A n-inch achromatic objective should be arranged in line, 

 so as to condense upon the object a fine image of the lamp flame. 

 In order to show that the image of the flame is accurately focussed 

 upon the object, a piece of wet tissue-paper may be laid upon the 

 top of the slide, or the image of the flame upon the face of the 

 observing lens may be viewed through a side facet. 



Under these circumstances the lines will be perfectly resolved 

 if the lens have an adequate angular aperture and he properly 

 adjusted. The method is very simple, but for want of it I have 

 seen an experienced manipulator spend hours in '' fiddling about for 

 the lines," and utterly exhaust his eyes without determining 

 whether or not the optical capacity of the lens on trial was at fault. 

 By the method I have described, this difiicult object may be re- 

 solved as easily as a Podura scale. If, when the lines are properly 

 resolved, the eye-piece be taken out, there will be seen, on looking 

 down the tube, at the southern edge of the field, a small clear 

 image of the flame, and at the northern edge — diametrically oppo- 

 site — a soft, greenish-blue diffraction image. Sometimes also an 

 outline of the diatom crossing the field from one image to the other 

 may be discerned. 



The particular angles given to the prism now before the Society, 

 were selected in order to enable a single prism to command the 

 whole range of oblique illumination, and to enable so difficult an 

 object as Amjjhipleura ]jeUticida to be at once resolved whether in 

 balsam or in air, and whether upon the slide or upon the cover. 

 Through these facets, light at somewhat different angles may be 



