:216 LT.-COL. GIFFORD 



The following papers are extended descriptions of 

 .exhibits shown before the meeting. 



NOTE ON LIGHT FILTERS FOR THE MICROSCOPE AND 

 PHOTOMICROGRAPHY. 



By Lt.-Col. Gifford. 



As far back as 1894 it was found that a solution of malachite 

 green in glycerine absorbed all the visual spectrum except a broad 

 band in the region of the F line of the solar spectrum, and a 

 narrow red band near B (J.R.M.S., 1894, pp. 164-7), and that such 

 a solution placed in a glass trough was eminently effectual as a 

 light filter for microscopic use, especially when the red band was 

 removed by inserting a piece of signal green glass into the fluid. 



The year following, a screen similarly constructed, but with a 

 solution of methyl violet for use in photomicrography, was described 

 (J.R.M.S., 1895, pp. 145-7). In recent years it has been found 

 that peacock-green glass possessed the same properties as signal 

 green to a greater extent, and the use of the latter has therefore 

 been dropped. 



Instead of the glass trough with the signal green placed in it, 

 the form these light filters have recently taken is as follows : — Discs 

 of peacock-green glass about 0.06 in. thick and of diameter to fit 

 into the substage condenser are cut out. On these discs are built 

 up cells, using gold size and soft metal rings, or the former alone, 

 just like those made for mounting microscopic objects in fluids. If 

 a metal ring is used, then a coat of size must be given to the top 

 of it and be allowed to dry. Then a final coat must be placed 

 on that and allowed to get tacky. Then place rather more than 

 sufficient dye solution in the cell near the edge. The glycerine 

 will cause it to stand up beyond the top of the cell. Take a clean 

 glass cover, make contact with the tacky gold size at a point 

 nearest the drop of dye. The point of contact will act as a hinge. 

 Now allow and assist the cover to fall until in contact with the 

 gold size all round the ring. While the cover is falling and this 

 contact is extending, the dye solution will flow forward and out 

 in a wave. When contact is made with the top of the cell all 

 round, take any blunt instrument and press the cover down in 

 the middle until still more dye flows out. While this is being done 

 adjust the cover on the ring if necessary, by the direction of the 

 pressure. Quite a considerable pressure may be used, cover glasses 

 are very flexible. When enough dye has passed out to leave the 

 cover slightly concave, and you are assured that the adjustment 

 •is correct, suddenly remove the pressure. Pneumatic action will at 

 once take place, owing to the resilience of the indented cover, and 

 the edge of the latter will adhere so tightly to the gold-sized ring 

 •that it is possible to wash under the tap with a full stream of 

 water at once. If made as described, the cell will not give out. 

 Light filters made in this way are shown; one of them has been 

 made and used for more than 20 years. 



