80 BANKS. 



As the name would suggest, this cell enables the worker to observe 

 all sides of an object parallel to its longitudinal axis, provided this cor- 

 responds with the longitudinal axis of the cell itself. For example, in a 

 preparation of the male genitalia of any species of mosquito, if the last 

 three abdominal segments be mounted in this cell, one may obtain both 

 lateral, ventral and dorsal aspects. The only aspect not possible would 

 be the caudal, and this, for anatomical purposes, is quite unnecessary, as 

 all essential features are demonstrable in the other four. 



The potyscopic cell, which is really nothing more than a section of 

 glass tubing of small caliber, is made by grinding it to the form of a 

 square prism instead of a cylinder. This, as I shall describe in detail, 

 is easily accomplished in any institution having a rock grinding ap- 

 paratus; and three or four dozen of these cells can be prepared in a day 

 by the plan which I have adopted. 



METHOD OF ' PREPARATION". 



Lengths of glass tubing of small diameter, say 4 to 6 millimeters, and of the 

 required caliber, are cut up into pieces of the desired length, usually 15 to 20 

 millimeters. It would be better to keep pieces cut from the same tubing together, 

 as these are most likely to have the same caliber and the cells will be kept uniform 

 in this way. Nine to a dozen of these short tubes are fastened to a small plate of 

 glass such as is used in grinding rock specimens. 



The best preparation for cementing to the glass is a mixture composed of 20 

 parts of white shellac and 7 parts Canada balsam. This, in the form of a pencil, 

 i? applied to the glass plate held over the gas flame, until a sufficient quantity has 

 melted upon the plate. The short tubes, are then placed close together and 

 pressed down upon the plate so that they will all be parallel. (See Plate I, fig. 2.) 

 The cement having become hard, the tubes are ground down upon the steel wheel of 

 a rock-grinding machine, the operator employing first coarse emery and then 

 finer until their surfaces have become worn to the desired degree and have the 

 velvety appearance of groimd glass. A still finer polish may be obtained by next 

 grinding for a short time on a plate glass with putmice and water. (See Plate I, 

 fig. 3.) 



The next step is to dry the plate and gently heat it until the tubes become loose 

 enough for removal. The entire mass of adherent tubes may be slipped off, turned 

 completely over, pressed firmly to the glass plate to remove air bubbles and, after 

 cooling, the operation of grinding the faces on the opposite side begun. (Plate I, 

 fig. 4.) This being completed, the tubes are now removed as before, set up on 

 edge so that their plane faces are contig-uous, recemented to the plate and the 

 third face ground. (Plate I, fig. 5.) For the fourth face, the mass may be 

 slipped off entire and turned over, the same precautions being taken to press the 

 mass flat to the plate. (Plate I, fig. 6.) The finished cells will then appear as 

 in Plate I, fig. 7. 



The cells may now be removed from the plate and, after cleaning off the 

 cement, they are ready for use. They may, however, be polished even more 

 finely if it is so desired, to remove the ground surface and render them perfectly 

 transparent like ordinary glass slides ; but this is hot absolutely necessary, for 



