ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



961 



upper circular plate can be rotated by the hand, or by turning the 

 small handle shown on the left it is fixed, and then can only be 

 rotated slowly by the adjacent tangent screw. The graduated semi- 

 circle and forceps are screwed to the upper plate, and can be removed 



Fig. 157. 



so as to leave the stage free. The movement of the index and forceps 

 jointly is effected by the larger milled head on the axis of the forceps. 



Osborne's Diatomescope. — Lord S. G. Osborne has forwarded to 

 us his diatomescope, together with notes from which we have made 

 the following description and diagram. 



The apparatus (fig. 158) consists of a rectangular brass plate, 3 in. 

 by If in., in the centre of 



which is a plano-convex Fig. 158. 



lens G, with its plane face 

 nearly flush with the upper 

 surface of the plate. A thin 

 metal disk C, having a 

 central opening correspond- 

 ing with the diameter of 

 G, is placed beneath the 

 plate to separate the upper 

 from a second plano-convex 

 lens F, which is mounted 

 slightly out of the centre 

 of the thick metal disk D, so that its axis B is a little at one side of 

 the axis A of the lens G. The upper surface of D is grooved to 

 permit a diaphragm-plate E, having a small square opening of 

 1/32 in., to traverse between the lenses. 



The apparatus is placed upon the stage of the Microscope, and 

 the slide is laid flat on the plate, with or without immersion-contact 

 with G, and is held in position by two spring clips. A pencil of light 

 suitably incident on F is refracted through the small opening in E 

 to the curved surface of G, where it is again refracted, emerging from 

 the plane surface more or less obliquely, according to the original 

 incidence and to the position of the diaphragm. 



We understand from Lord S. G. Osborno that the device has also 

 been made to fit in an ordinary substage, where it is of course more 

 serviceable than on the stage proper. 



