2 THE AMERICAN MONTHLY [Jan 



mination particularly calculated to show up numerous 

 classes of objects to great advantage, and which at the 

 same time give very pleasing effects. 



It is now some five years since I first began to make ex- 

 periments with a view to finding out how it might be pos- 

 sible, without staining, to cause an object and its back- 

 ground to appear of different colors, and so secure a greater 

 contrast than usual. Up to the present I have found three 

 different ways by which we can make uncolored objects 

 assume any color we wish, and our background any other 

 color. In many circumstances we can also make definite 

 parts of objects themselves assume different colors — if so 

 desired. These effects may be produced so simply that 

 any amateur may make many of the experiments himself 

 at the cost of a few cents. 



In all microscopes fitted with a condenser in the sub- 

 stage, there is, underneath the condenser lenses, a ring or 

 some form of holder to take stops for dark-ground illu- 

 mination. Now let us cut out a disc of red gelatine (such 

 as is used for crackers) to fit this ring, then punch a hole 

 in the centre about a third of its diameter, aud stick over 

 the hole a piece of blue gelatine of the same size (fig. la). 

 Then we place this color disc in the holder under the con- 

 denser, and use it in the same way as we would use the 

 dark-ground stop. We will suppose we are uging a 1" 

 objective. The result to those who have not seen it be- 

 fore will be astonishing. The objects, for instance a slide 

 of Polycystina, or some living Rotifers, will appear per- 

 fectly red, and the background perfectly blue. The great 

 contrast throws the objects up in a most striking manner. 

 Of course, if we wish to vary our colors, all we have to do 

 is to vary the colors of the gelatine ; a yellow disc with a 

 blue centre will show the objects yellow ona blue ground, 

 an uncolored disc with a green centre (fig* 1 b) will show 

 the object white or whatever may be its natural color on 

 a green background, and so forth. We must take care, 



