414 OPTICAL PROJECTION 



this way, the Indian-ink lines being laid in as fine and clean 

 as if printed from a steel plate, To do this, however, the 

 plate must be perfectly free from the least greasiness, and it 

 helps a little to put a very little ox-gall in the last wash of 

 water given to it. Water-colour may be laid on also if 

 desired, or pencil shading be added to the ink lines. When 

 the drawing is finished, it is held at one corner, and a varnish 

 of dried Canada balsam dissolved in benzol flowed over it. 

 The varnish should be about as thick as cream, but the exact 

 consistence does not matter much. Pour it on freely so as 

 to cover the whole plate quickly, pour off at one corner till it 

 nearly ceases to drop, then cant that corner up to flow rather 

 back again, and then cant other sides up a little : a very few 

 trials will teach the operator to do this so that the varnish sets 

 level, and free from ridges or waves. Then protect from 

 dust and leave the varnish to dry ; all the grain of the glass 

 will disappear, and the slide be perfectly transparent. When 

 dry, it is mounted with a mask and cover-glass as in 248. 

 Anyone with the least capacity to draw upon anything, can 

 produce beautiful slides in this way with the greatest ease ; 

 and the process is equally adapted for simple line diagrams, 

 or the most finished representations of microscopic detail. 



247. Printed Diagrams. It may often happen that a scien- 

 tific lecturer has at his command, through acquaintance with 

 a publisher, or perhaps because the blocks belong to his own 

 works, wood engravings which are exactly suitable to illustrate 

 his subject, not only in detail and treatment, but also in size. 

 It is well to know that such may be converted into excellent 

 lantern slides, by a simple method which occurred to myself 

 quite accidentally. I had collected or prepared, as I thought, 

 all the slides for a lecture I had promised to give, when I 

 came across several engravings I was very desirous to intro- 

 duce, only the very day before the date. There was no time 

 to photograph them, or to attempt imperfect copies ; but it 

 Suddenly occurred to me to ask the printers for proofs on 



