ZOOLOGY AND BOTANY, MIUKOSUOPV, KIG. 325 



it is desired to scatter. Tlie length of the piece of springy steel may 

 conveniently be made to be about 1 in., so that it will just reach across 

 an ordinary 8 by 1 in. glass microscope slide. Bind the slide in a piece 

 of thin metal having a |-in. perforation at the back— that is to say, so 

 bend a piece of thin sheet metal that an ordinary slide will slip into it 

 through grooves along the two sides of the folded piece of metal. (See 

 fig. 5.) This metal should simply pass around the edges of the slide 

 and lap over about a sixteenth of an inch at each edge leaving one face 

 of the slide uncovered. The grooves should be a little wider than the 

 thickness of the slide — at least enough wider so as easily to admit the 

 thin i^erforated metal spring. Place the cells, the chromosomes of 

 which are to be studied, on the slide opposite the middle of the f-in. 

 aperture. Use very little mounting medium ; cover the cellular tissue 

 to be treated with a- small round cover-glass. Tuck the ends of the 

 bowed piece of springy perforated steel under the edges of the metal 

 slide-case or holder, holding the spring against the small cover-slip in 

 such a way that the cells to be compressed lie opposite the centre of the 

 small perforation. Press and lock the spring in the same way as in the 

 case of the springs at the back of an ordinary photographic printing 

 frame. The cells will now be under pressure at or near the centre of 

 the perforation in the steel spring. The entire contrivance will differ 

 but very little in form and size from an ordinary microscope slide, and 

 can be placed on any microscope stage iu the same way as a slide. The 

 piece of springy steel is so thin that it in no way prevents the use of 

 a high-power immersion objective. Needless to say. it is for this reason 

 that it is made from such thin metal. The spring may be manipulated 

 with the aid of matches or wooden toothpicks. 



Ordinary slides and cover-glasses are almost never perfectly flat. 

 Better results will be obtained by this method if the slide has its 

 convex surface up and the cover has its convex surface down, so that 

 the cellular tissues to be treated lie between two very slightly convex 

 surfaces. It will be found that in this way very compact groups of 

 chromosomes and other similar objects can sometimes be scattered so 

 as to be counted, when otherwise they could not be counted. 



There seems to be comparatively little danger of exerting too much 

 pressure. The beginner's tendeucj at first is to exert, if anything, 

 too little pressure. The greatest difficulty arises from sliding the 

 glasses on each other, since much of this ruins the preparation. To 

 overcome this difficulty, a series of three or four notches, close together, 

 may be filed in the edges of the metal holder before it is folded about 

 the slide — or rather about the metal core on which it is bent, or 

 formed, and which naturally has a little greater width and thickness 

 than the slide. If now the bowed string has a length a little less than 

 the distance between the bottoms of the notches in the edges of the 

 slide-holder, it will be found when it is pressed down that the pointed 

 ends can be tucked through the notches and under the edges of the 

 holder without materially sliding or rotating the spring. The accom- 

 panying illustrations will assist in understanding this simple and 

 effective device. 



The particular cells to be compressed are prepared and searched 



