144 THE AMERICAN MONTHLY [June, 



it will aflbrd hiin occupation for a long while, for Ehrenberg 

 found no less than 282 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 500. 



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 i-4th, i-5th, i-6th. Then use direct light from a 

 flat wick, dispensiiig with the mirror and using a binocular 

 diaphragm, or, in case of a i-4th, i-5th, or i-6th obj., a dia- 

 phragm with round opening just large enough to fill with light 

 the back lens of the objective, as mav be ascertained by removing 

 the eye-piece and looking down the right-hand tube after shutting 

 ofl' the light from the left, and by all means d ijf use ihc \\g\\i. 

 For this you can use ground glass in your diaphragm ring, but 

 there is something equally good, if not better, which you can 

 make for yourself tor nothing. Take a thin piece of mica and 

 rub it on both sides with emery paper in tw'o 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 efiect. 



Then when you have finished with the binocular examination 

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

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

 1-2 to the 1-4, 1-5, 1-6, and even i-S, using every now and then 

 your diaphragm of mica to difflise the light, for it often brings 

 out minor points ber.utifully. If you have never tried these 

 higher powers on the polycystina you will be astonished and de- 

 lighted 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 diam- 

 eters you will admit that there is good reason for what I urge, 

 or, if not, a glance at the figures themselves will speedily con- 

 vince 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 polycistina, beyond such a superficial 

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

 became enthusiastic, and I am confident you will also. And you 

 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 Moller'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 tell 

 whether he ever made more than one. In fact, as I understand 

 it, none of those slides are now in the market, nor will they be 



