1900] MICROSCOPICAL JOURNAL. 5 



plicable to all objectives, no matter of what power they be. 



We will suppose we are looking at some diatoms with a 

 2k" objective, and we will put a color disc below the con- 

 denser as before, one with a red centre and a green rim 

 this time. The red centre should be of ruby-colored gel- 

 atine, which can be easily obtained, but the particular green 

 gelatine for our purpose is difficult to get, and the best 

 thing to do is to buy a little of the stain known as mala- 

 chite green, dissolve it in alcohol, and then add a little of 

 the dissolved stain to some collodion. A little of the dyed 

 collodion should then be poured over the ordinary green 

 gelatine, which can be bought ready. It quickly evapo- 

 rates, leaving a thin film of the gelatine of a beautiful 

 blue-green color such as we require. 



It is a well-known fact that white light is made up of 

 light of all colors, and that by means of a prism the rain- 

 bow colors can be re-combined to form white light. It has 

 also been practically demonstrated by Ives, the iuventor 

 of the photochromoscope, how the so-called fundamental 

 colors, viz , green, red, and blue-violet, can be re-com- 

 bined so as to appear white. But it is not so generally 

 known that, if correctly chosen, two colors only are need- 

 ed to give the impression of white light to the eye, when 

 combined in the proper ratio. Such however is the fact, 

 and this is the principle we are going to work with. If we 

 can mix the ruby-red light of the central portion of our 

 color disc with the green color of the rim in the right pro- 

 portion, we can obtain light the color of which to our eye 

 appears almost white. Now the mixing of these colors is 

 exceedingly simple, provided our condenser has an iris 

 diaphragm to it. 



Let us look down the microscope with the iris quite open, 

 then the field or background appears quite green, for this 

 time we are using an objective of wide aperture, one 

 which will admit a cone of light much wider than that 



