New Tank, and Pond-weed Holder. 535 



observation is very essential, and a tank with a large open mouth 

 provides for this, but it can only be used in conjunction with low- 

 power, immersed objectives. The immersion objective used on the 

 Green ough Microscope is a Leitz 24 mm. According to the 

 maker's list this has an initial magnification of 8 • 14 diam., and 

 eye-pieces have been provided giving up to 170 diam. The con- 

 denser employed is one by Swift, which has a long working distance, 

 and is specially useful for pond-life. 



To give an idea of the kind of image that can be obtained, using 

 dark-ground illumination, I have been able to display the cilia of 

 a small alga, Denohryon cylindricum, at a magnification of 132 diam., 

 and the definition was quite good. To see Volvox in stereoscopic 

 relief, with cilia actively working, is an impressive sight, and the 

 same might be said with regard to the crisp and bright image given 

 of fixed forms, such as Melicerta and Floscularia. 



When working with low-power immersion objectives in an 

 uninterrupted medium for which they have been corrected, one 

 is employing ideal conditions for the proper presentment of 

 an object, and the results obtained with pond-life encourage the 

 hope that something will be done to produce, say, two powers — 

 §-in. and ^-in. immersion objectives* — to be employed with the 

 monocular. The Zeiss Plankton Searcher, which is the only low- 

 power water-immersion objective I am familiar with, gives an ex- 

 cellent image, but is not a convenient lens to handle ; the working 

 distance, 36 mm., is too long, and it yields a low magnification only. 



I have already worked out a tank and weed-holder for the 

 monocular, and the dimensions will be different from those of the 

 tank for the Greenough, owing to the somewhat limited working 

 space on the former instrument. It can, however, be put to the 

 same use as the binocular tank, with an additional advantage of 

 holding in a fixed position small free-swimming forms between two 

 glass covers — in the same manner as employed with the Rousselet 

 live-box. 



* With working distance of 25 mm. and 13 mm. respectively. 



EXPLANATION OF PLATE XIX. 



A. — Shaft which slips into the holder H on the triangular metal plate E. 



B. — Solid metal drum, with two holes bored through ; into one hole fits the 



shaft, the other takes the peg on drum C. 

 C. — Solid metal drum, with one hole bored through ; this takes the peg of the 



duck-bill clip D. 

 D. — Duck-bill clip and peg ; the clip revolves and can be placed in a position 



either perpendicular or horizontal to the bottom of the tank. 

 E. —Triangular metal plate, on which is the shaft holder H and two hooks ; the 



hole at the apex takes the metal screw P. This part of the weed-holder 



is in one piece. ~ 

 F. — Screw which controls position of the duck-bill clip, moving it either towards 



or away from the back of the tank when the holder is in position for use. 



