COLLECTING AND MOUNTING ROTIFER A. 323 



settled down and the water had become sufficiently clear to be 

 examined with the hand lens. The aquarium having been placed 

 on a whatnot or dinner-waggon before a window or in front of 

 a lamp, candle or electric light, all the free-swimming Rotifera 

 collected on the side nearest the light and could then be picked 

 up by the score or hundred with very little dirt by means of a 

 pipette fitted with an indiarubber teat, and transferred to one 

 of my solid w^atch- glasses,* which being placed on the stage of 

 my Stephenson binocular enabled me to examine them under a 

 low power (3 inches). 



Any form unknown or new to me could then be picked out 

 and transferred to the micro-glass trough or to my live-box or 

 screw compressor for further examination with the IJ incli or 

 higher power, and under dark-ground illumination. 



After this preliminary selection the Rotifers in the solid watcli- 

 glass were first of all narcotised with 1 per cent, euca'ine and then 

 fixed and killed with one drop of J to J per cent, osmic acid, left 

 there for one minute only and then removed by means of a pipette 

 to several changes of 2 J per cent, formalin, in which they were 

 finally mounted. Formerly I recommended w^ashing in distilled 

 water, but the difference in density caused some species to swell 

 too much. This swelling can be controlled and regulated by 

 increasing or decreasing the percentage of formalin. 



The 1 per cent, solution of osmic acid sold by opticians is as a 

 rule very much weaker than its nominal strength, as the acid 

 evaporates and gradually disappears ; moreover a 1 per cent, 

 solution will blacken and spoil the specimens if they are kept in 

 it even a very short time. For this reason and so as to know 

 the actual strength of the solution I was working witli, I found it 



* In the earlj'^ days my friend ]\Ir. Dixon Xuttall had cut for nie 

 by one of his friends a number of squares of thick i^late glass, 1| to 

 2 inches square, and varying from f inch to J- inch in thickness. In 

 the centre of each square of glass I caused to be ground a cavity more 

 or less deep. These solid watch-glasses, as I called them, could not 

 only be placed singly upon the stage of my dissecting mici'oscope, 

 but when not so used could be placed one upon the other to protect 

 the contents from dust, etc. 



JouRN. Q. M. C, Series II.— No. 81. 25 



