19()0J MICROSCOPICAL JOURNAL. 173 



of barium for ten minutes. Transfer it to a bath of dilute 

 sulphuric acid and gently rock the Boiution to and fro un- 

 til a fine, even precipitate of barium sulphate has been 

 deposited. Wash and dry the plate, and it will be ready 

 for use. 



Focussing. — Find the centre of the ground glass screen 

 and then place a large circular or square cover-glass on 

 it with Canada balsam. To do this, warm the ground glass 

 carefnlly, add a drop of rather thick balsam to the centre 

 on the ground side, and then apply the cover and press 

 it down firmly. Put it away on a warm shelf for a few 

 days to harden, after which the excess of balsam may be 

 removed from the edges with the aid of a penknife and 

 xylen or alcohol. The balsam will fill up the inequalities 

 in the glass, and being of about the same refractive power 

 will make this part of the glass clear as if it were ungrouud. 

 The focussing screen as thus prepared with a clear cen- 

 tre, serves both for the general focussing and the finest 

 focussing,and avoids the danger of using the double screen. 



Opaque Objects. — For the photography of opaque sub- 

 stances, such as metals, &c., a metal microscope, such as 

 that which is made by Reichert, of Vienna, is necessary. 

 The microscope must be fixed in an upright position, and 

 reflected light used. The source of light should be at a 

 distance of one to one and a half metres from the appa- 

 ratus, and must be on the same level as the reflecting lens 

 at the side of the vertical illuminator on the tube of the 

 microscope. The specimen should be everywhere equally 

 illuminated and then focussed. Eosin plates and the use 

 of a yellow screen are to be recommended for this work. 



Plates. — The selection of plates and screens for photo- 

 micrography is of practical importance, and should re- 

 ceive careful attention : — For stained preparation ortlio- 

 (ihromatic plates give the best results, but it is of advan- 

 tage to place a screen, complementary in color to the stain 



