1900] MICROSCOPICAL JOURNAL. 7 



to what extent its aperture or area is filled with light. 



Now we will proceed to the third method of multiple 

 color illumination — one which depends upon a quite dif- 

 ferent set of principles to the other two, viz., on the class 

 of phenomena known as diffraction. But, as before, we 

 will let practice precede explanation. We take some small 

 microscopic cover-glasses of the thinnest kind, choosing 

 them of such a size that we can drop them from above on 

 to the back lens of the objective which we are about to 

 use, say a &" or \" objective. They must, of course, com- 

 pletely cover the back lens. We transform these little 

 cover glasses into color discs by coating them with stain- 

 ed collodion, for gelatine such as we used for the discs 

 placed under the condenser is not nearly sufficiently clear 

 and homogeneous for the present purpose. The best way 

 to make the stained collodion is to dissolve the dye (fuch- 

 sine, methylen blue and malachite green are suitable 

 dyes) in pure alcohol, and then add it to the collodion, 

 which may be bought ready. I would, however, strongly 

 recommend those who wish to make the experiments also 

 to make their own colodion by dissolving a little of Scher- 

 ing's celloidin in equal parts of ether and alcohol. This 

 gives very much better results than the ordinary ready- 

 made collodion. With a pipette or a glass rod we drop 

 the dyed collodion on the little glass circles and let it 

 evaporate, which it does very quickly. Then with a needle 

 we scratch the film off the glass except where it is requir- 

 ed. If we want to make a red disc with blue centre, we 

 coat the one side of the glass with a red, the other side 

 with a blue film. All the blue excepting a small central 

 spot about one quarter the diameter of the top lens of 

 the objective is then removed, and on the red side the film 

 is scratched away from the central area to correspond. 



Having dropped our little disc into the objective, we first 

 focus the latter on to our object, a section of bone, let us 

 say, or if preferred, we may look at diatoms again, as 



