ZOOLOGY AIsD BOTANY, MICROSCOPY, ETC. 523 



reached long before one got to the pinhole cap, which opticians 

 supply to condensers for centering purposes. 



This, then, is the 'sliding stage diaphragm' I have devised, and 

 which I think I may claim as original as it is economic and simple. 

 In my hands it has assuredly improved even the images of objects 

 seen with the fine -^^ of Tolles and Zeiss, while with a fine ^ and 

 deep single ocular it has produced such vivid images and shown such 

 detail as almost to suggest the self inquiry of ' What more one 

 wants ? ' May one trust, then, that this simple bit of apparatus will 

 be a boon to all students of nature through the Microscope ? Of 

 course it could be made of various substances. Ebonite suggests 

 itself as appropriate and sightly, while to metal there would be 

 several objections. 



In looking to the classes of objects to which this diaphragm would 

 be suitable, those requiring ' resolution ' must be at once put on one 

 side, and the advantages looked for in improving the defining power 

 of objectives on the thousand and one objects which interest the micro- 

 scopist, and these advantages can easily be verified on reticulated 

 structure, and indeed on any delicate tissue apt to be half lost in a 

 flood of light. 



Fine as is the image shown by the stage diaphragm in an appro- 

 priate object, a finishing touch, very slight, to the Iris diaphragm will 

 often bring about a still further perfection of image, which the prac- 

 tical microscopist will not be slow to appreciate." 



Bousfield's Rotating Diaphragm-plate.* — Mr. E. C. Bousfield 



describes a rotating diaphragm-j^late, shown in Fig. 123 (reduced size), 



which he has devised for use close beneath the object on the stage. 



It consists of a brass plate A, about 



-^ inch thick, fitting on the stage in ^'^1 '"^' 



the manner of a super-stage by means 



of two pegs let into holes in the stage 



itself. A hole C C, about f inch in 



diameter, is bored through the plate, and 



over this revolves a plate B about 



2 inches in diameter, or of such a size 



as to project beyond the outside of the 



plate A, and of the central hole. The 



edge of this plate is milled, and it is 



about -^-g- inch thick ; it is suak into the 



face of plate A just sufficiently to allow 



of its being turned when the slide is in 



place. The apertures in this smaller plate vary in size to suit the 



different sizes of field seen with objectives of difierent focus. D D 



are grooves for the usual sliding object-holders. 



Mr. Bousfield, in view of the fact that diaphragms should always 

 be placed close under the object, notes that "Iris" diaphragms (as 

 usually made) fail in this respect, as by the system of construction 



* Engl. Mech., xxxii. (1881) p. VJo. 



