SCIENTIFIC DIAGRAMS 415 



thin clear sheets of gelatine. These were taken for me, the 

 gelatine used being very thin, such as is used for wrapping 

 up Christmas crackers, but uncoloured. The printer's ink 

 we found should be rather stiff and fine : with such ink, and 

 what is called a ' hard ' impression, the result was perfectly 

 satisfactory, and the slides, prepared in an hour or two, were 

 as good as photographs. 1 I have since resorted to the same 

 method on several occasions, and whenever there is available 

 material, can thoroughly recommend it as rapid, inexpensive 

 and perfectly satisfactory. The gelatine film appears after a 

 little to settle into ' crinkles ' between the protecting glass 

 plates ; but this is of no consequence, and in no way affects the 

 appearance upon the screen. The only objection is an almost 

 imperceptible tendency to a slight thickening of the lines. 

 By those at all familiar with mica work for the polariscope, 

 even this and the ' crinkling ' may be avoided, as I have 

 myself since avoided it, by splitting very thin films of mica, 

 cutting them into 8J-inch squares, and taking the proofs upon 

 these. 



Even proofs upon some samples of very thin tissue-paper 

 (as much freedom from ' grain ' as possible is the main thing) 

 will make passable slides for a sudden emergency, if the proof 

 is afterwards fastened to a glass plate by the balsam and benzol 

 varnish already mentioned. This makes the tissue nearly trans- 

 parent, and the amount of grain visible in the paper is the 

 only defect, which with some papers is very slight. 



248. Mounting Slides. What are called ' masks should 

 always be used in mounting slides ; that is, circles, ovals, or 

 cushion- shapes cut out of the centre of 3|-inch squares of 

 thick black paper or thin card. Such can always be obtained 

 of opticians or lantern-dealers, and a mask should be selected 

 to suit the diagram. A drawing on glass (all glasses being 8| 



1 Merely to indicate the class of illustrations, I may mention that the sub- 

 ject was ' The Eye as an Evidence and Example of Evolution,' and the 

 diagrams exploited related to points of histology and comparative anatomy. 



