142 PRACTICAL TREATISE ON THE MICROSCOPE. 



Powell also secures his plate to a large stage by means of a 

 spring clip, whilst that represented by fig. 95 is provided 

 with a slightly conical brass pin, made to fit into one of 

 the holes of the object-plate, and on which it is capable of 

 being revolved. At the base of the pin there is a small strip 

 of brass for securing either the tape or string attached to the 

 bag containing the frog. Some persons employ a piece of 

 cork or soft wood in preference to the brass plate ; this has 

 many advantages, and will be again alluded to in the chapter 

 devoted to the most approved methods of exhibiting the 

 circulation of the blood in the lower animals. 



Fish Troughs. — From the time of Leeuwenhoek to within 

 the last few years, all microscopes of any importance were 

 supplied either with a glass tube or a fish-pan for holding 

 small eels or minnows, in order that the circulation in their 

 transparent fins might be seen ; these have aU given place to 

 the frog-plate just described; but when it is required to 

 exhibit the circulation in the taU of a small fish, a glass cell or 

 trough win be found very convenient. This should be a little 

 deeper and longer than the fish itself, and the fish should be 

 secured in it by a broad tape or bandage, wound loosely round 

 the middle third of the body, or even carried down to within 

 a very short distance of the commencement of the tail. In 

 order to keep the fish alive, the bandage should be wetted or 

 the trough fiUed with water ; and to prevent the fiapping of 

 the tail against the object-glass, or the. condensation of the 

 aqueous vapour upon it, the end of the cell where the tail is 

 may be covered with a piece of thin glass. The author uses 

 a cell constructed after the plan shown by fig. 96, which an- 

 swers the purpose very 

 ^""flSfHiliryif^rT^taSHin well, a represents a 



plate of glass about 

 three inches in length. 

 Fig. 96. ^ ^ glass cell cemented 



to it, c one of two pieces 

 of glass to raise the bottom-plate above the level of the stage, 

 in order that the bandage, d, may lie in a cavity, and not pre- 

 vent the trough from resting perfectly horizontal ; e is a thin 



