INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 771 



length, so that it can be pushed to any desired position without dis- 

 turbing the position of the central steel rod (10), at the upper end of 

 which a lens (9) is fastened. The lens (9) is such a segment of a 

 hemisphere of crown glass, that when brought into optical contact (by 

 oil of cloves) with the under surface of an ordinary glass object-slip, 

 the object to be studied will be as nearly as possible at its centre of 

 curvatiu'e, and the rod (10) slips freely in the top of the substage 

 tube (8), so that the lens may be pushed into position or withdrawn at 

 pleasure. 



In using this ajiparatus with monochromatic sunlight, I first set the 

 square brass plate )3) at the desired angle as read on the quadrant, and 

 then slip the transverse bar (1) backwards or forwards as may be 

 necessary, until the pencil of monochromatic sunlight (to which the 

 desired degree of obliquity has been previously given by means of a 

 prism) falls centrally through the slip-tube (4) and illuminating 

 objective (5) upon the face of the lens with which the object is viewed. 

 By means of the slip-tube, the illuminating objective (5) is then 

 brought to the proper focal position. Ordinary illumination is thus 

 obtained of any desired obliquity, from about 30'' to the limit of the 

 thickness of the stage. When I desire still greater obliquity I use 

 Powell and Lealand's extra stage, and slip the transverse bar into 

 the groove at the upper end of the holder which those makers 

 provide with it to carry the small bull's-eyes they furnish for the 

 examination of Amphipleura pellucida. In this manner I can get more 

 oblique illumination up to 80^ or even 85°, but of course the oblique 

 pencils thus obtained are refracted at the under surface of the glass slip 

 that carries the object, and cannot possibly reach the object itself at an 

 obliquity greater than 41°. To obtain greater obliquity than this, I 

 make use of the hemispherical lens (9). The illuminating objective 

 is set at the desired angle, say 45°, and the object illuminated as 

 described above. When this is satisfactorily done a droj) of oil of 

 cloves is placed on the flat surface of the hemispherical lens, which is 

 then pushed up into contact with the under surface of the slide on 

 which the object is mounted. The light now enters in the line of a 

 radius of the hemisphere, at the angle registered on the quadrant (7). 

 Fig. 6 represents a section of the apparatus when thus in use (also 

 slightly reduced in size). The numbers in the two figures correspond. 

 In addition, on Fig. 6, A is the objective, B the slide carrying the 

 object, and C the immersion fluid. 



I have found this apparatus exceedingly convenient for the pur- 

 poses of photo-micrography and sunlight work generally ; for when I 

 have once obtained any particular result by means of a certain obli- 

 quity, I am able to reproduce the effect at pleasure without any loss of 

 time. It has also proved useful, for the same reason, by ordinary 

 lamplight. When, however, the object of the microscopist is merely to 

 resolve Amphipleura peUndda or similar tests mounted in balsam, by 

 lamplight, with suitable objectives, I still give preference to the 

 simple substage prism I described last year,* through which I can 

 throw the light at once at an angle of 45° by means of the concave 

 * See this Journal (1878), p. 246. 



