MICROSCOPICAL TECHNIQUE. 85 



hours in an oven at a temperature below the melting point of 

 gelatine. Should any air-bubbles have got between the retina and 

 the glass, they must be removed by pressure with the camel-hair 

 pencil before the specimen is dried. The retina is then clarified 

 by allowing it to remain for two or three days under oil of cloves, 

 until all opacity is removed. The clove-oil is drained off, and the 

 retina covered with solution of balsam in benzole, and another thin 

 lantern slide used as a cover-glass. Should it be desired to take a 

 direct negative photograph of the vessels, this can easily be done, 

 before the clove-oil is removed, by placing the silver paper directly 

 in contact with the specimen and exposing it to the light. The 

 clove-oil, which will have sunk into the paper, can be removed 

 with methylated spirit, and the development proceeded with. 

 Specimens prepared in the above manner are equally suitable for 

 naked-eye and lantern purposes, and for microscopical examination 

 if sufficiently thin glasses have been used. 



Method of Killing Nematodes. — The following method for pre- 

 venting Nematodes from curling while being killed is recommended 

 by a writer in the November number of the Americaji Naturalist^ 

 who has found it indispensable in fixing Nematodes and other 

 worms : — 



" The worm is placed in a few drops of water upon a large 

 slide ; a second sUde is placed over the worm and moved slowly 

 to and fro. This movement causes the worm to straighten. As 

 soon as the Nematode assumes the desired position, the fixing 

 liquid is pipetted between the slides, the motion of the upper slide 

 being continued until the worm is dead. By this method one can 

 obtain a specimen which is perfectly straight and round. If the 

 worm is delicate, too much pressure must not be used during the 

 rolling process. Pressure may be avoided by pasting a piece of 

 paper on the upper surface of the second slide and using that as 

 an handle. As a killing liquid, the following solution is used : — 

 Corrosive sublimate ; alcohol, 70 per cent. ; and a few drops of 

 acetic acid, heated to 50^ C, which passes through the cuticle 

 very quickly." 



