178 INTRODUCTION TO ZOSlOGY. 



object (for example, the tongue of a fly) into strong 

 alcohol. This should be either absolute (one hundred 

 per cent is always preferred) or ninety-eight per 

 cent, or even ninety-five per cent. It should then 

 be placed on a glass slip 1x3 inches and covered 

 with clove oil. In a few minutes the excess of clove 

 oil should be removed with a blotter, and the object 

 covered with a drop of Canada balsam made liquid 

 with xylol. A cover glass is then placed on the slide, 

 with the object iu the center, and pressed down with 

 a clamp like a spring clothes pin. In a few days the 

 balsam near the edges will be dry, and any excess 

 may be scraped off with a knife. The principal dif- 

 ficulties will be to use alcohol strong enough to 

 remove all the water, and to avoid the inclusion of a 

 bubble of air in the balsam. 



If only one compound microscope can be had, it may 

 be made very serviceable to a whole class by means 

 of a porte lumiere. This is easily and cheaply 

 made as follows : Set a board twenty inches wide 

 across a window, under the sash. Cut a hole four 

 inches in diameter near one edge of this, and in this 

 hole mount a double convex lens having a focus of 

 about twelve inches. The lens is the condenser. 

 Directly under the lens, near the bottom of the board, 

 cut a slit large enough to allow another board six 

 inches wide and i inch thick to pass. Let this 

 transverse board extend about a foot outside the room. 

 Fasten it firmly. On the outer end of the board, 

 mount a mirror 9x12 inches in such a manner that it 

 can be turned in two directions. Small iron rods 

 running from the mirror frame to the inside of the 

 window board may be used to turn and hold it. 



The mirror is used to reflect sunlight upon the 

 condenser. This concentrates the rays of light to a 

 point at the focus. To take out the rays of heat, a 

 tank composed of two plates of glass separated at the 



