22 THE MICROSCOPE. [February, 



Before making an exposure there are several other points to 

 observe. 



i. A firm and steady support must be provided for our camera 

 board to rest upon. No jarring is to be permitted during the 

 exposure, as it will give a blurred image. 



2. The heat rays should be intercepted by placing between 

 the mirror and condenser, or mirror and object, a vessel with 

 glass sides and filled with the following solution : 



Sulphate of copper, . . . 175 parts. 



Bichromate of potassium, . . 17 " 



Water, ...... 1,000 " 



The vessel may be easily constructed from a pane of glass 

 with a glass-cutter. It may be four inches square, and one-half 

 inch between the glass sides. 



3. Some means must be provided for exposing other than 

 drawing the slide of the plate-holder, as it would disturb the 

 adjustment. In using a low-power objective with long-working 

 distance, a blackened card may conveniently be placed between 

 the object and the objective, thus intercepting the rays while the 

 slide of the plate-holder is withdrawn, when the card can be lifted 

 long enough to make the exposure, and then replaced while the 

 slide is returned in the plate-holder. Care should be exercised 

 even here, as when the sun is shining directlv uoon the objective 

 enough light may enter between the card and objective to 

 spoil the negative. Of course no light must enter at the point 

 where the draw-tube and camera are connected. But with an 

 oil-immersion lens a different arrangement must be made, and I 

 have found that a cardboard shutter in the end of the camera- 

 cone, just in front of the end of the draw-tube where they are 

 united, is very convenient. 



We have been supposing, in the above description, that sun- 

 light is to be used in our work, but if lamplight is to be used 

 any convenient shade may be placed between the lamp and the 

 condenser. 



Let us now describe the several steps in taking a photomi- 

 crograph. We will first light the ruby lantern, enter the dark 

 room, shut the door, unwrap a package of negatives, and, select- 

 ing one. we insert it, film side up, in the piate-holder. It should 

 be carefully dusted with the broad camel's-hair brush and the 

 slide inserted. With everything in readiness, with due attention 

 paid to the lighting of a good microscopical object and perfect 

 focusing of the image upon the ground glass, with the heat 

 intercepter in place, and the shutter or cardboard properly ar- 

 ranged, we draw the slide from the plate-holder and make the 

 exposure. How long shall it be? This is a perplexing ques- 

 tion, because so much depends upon the plates we are using, 



