112 THE AMERICAN MONTHLY. [May, 



there is in that deposit as he mounts one after another. If he is 

 at all thorough about it, it will afford him occupation for a long 

 while, for Ehrenberg found no less than 2S2 species and Mrs. 

 Bury added to this large list 141 more, while Haeckel says that 

 the total number of species in that deposit alone is probably over 



00. 



But whether one mount them separately or not let him not be 

 content until he has practically isolated each form by using a 

 power high enough to shut out almost all the other forms from 

 the field of view. Begin with the binocular and dark-ground 

 illumination, by means of the Abbe condenser, and carry the 

 powers up to J^th, 4th, 1th. Then use direct light from a flat 

 wick, dispenshig -with the 7nirror and using a binocular dia- 

 phragm, or, in case of a |^th, ith, or \\h obj., a diaphragm with 

 round opening just large enough to fill with light the back lens of 

 the objective, as may be ascertained by removing the eve-piece 

 and looking down the right-hand tube after shutting off' the light 

 from the left, and by all means difuse the light. For this you can 

 use ground glass in your diaphragm ring, but there is something 

 equally good if not better wliich you can make for yourself for 

 nothing. Take a thin piece of mica and rub it on both sides with 

 emery paper in two directions at right angles to each other. The 

 result is a fine mat surface, and this will prove just the thing to 

 bring out the forms with remarkable stereoscopic effect. 



Then when you have finished with the binocular examination 

 take the monocular tube and proceed as before, first with dark- 

 ground and then with direct illumination, and go on up from the 

 ^ to the :|-, -J. ^, and even ^, using everv now and then your dia- 

 phragm of mica to diffuse the light, for it often brings out minor 

 points beautifully. If you have never tried these higher powers 

 on the polycystina you will be astonished and delighted at the 

 beauty and variety of markings as so displayed, and when I tell 

 you that all of Haeckel's figures are drawn under a power of at 

 least 300 diameters and many of them of 400 diameters you will 

 admit that there is good reason for what I urge, or, if not, a 

 glance at the figures themselves will speedily convince you, if you 

 are fortunate enough to get a peep at them. They are simply 

 exquisite, and there is no end to the variety in the plates of that 

 magnificent work. I began this investigation without knowing 

 anything about the polycystina, beyond such a superficial knowl- 

 edge as is to be obtained from opaque mounts. But I soon became 

 enthusiastic, and I am confident you will also. And 3'ou will 

 surely say to yourself, as I have said, why in the world do not 

 preparers put up separate forms of the polycystina as they do of 

 the diatoms.'' Why cannot one buy type-slides of them.'' I see 

 by the catalogue of MoUer's diatoms that he has one slide of 72 

 forms of the polycystina, but they are mounted opaque, which is 

 not the best method for close study, and the catalogue does not 

 tell whether he ever made more tlian one. In fact, as I under- 



