239 



loop of thread marked h' is passed over the tail, and by pull- 

 ing its lower end h, is drawn down over the narrow part 



Outline Sketch of Tish-Trough on the Stage of the Microscope. 



of the tail ; by this means the tail is held still : l l are two 

 small springs, which press gently on the glass stage ; the tail 

 is now stretched to its natural width, and each extreme edge 

 placed under one of these springs. 



It is well to have a tap on the supply pipe ; one on the 

 waste-pipe is also useful, as thereby an unjjleasant bubbling 

 may be prevented. If the trough is in order, not a drop of 

 water escapes. The microscope should be incKned at an 

 angle of about 45°. 



A gentle stream of water being kept circulating, the fish 

 may be left in this position some hours without apparent 

 injury. Eight hours is perhaps the maximum. 



In a suitable fish, free fi'om parasites, the tail membranes 

 are as clear as glass. They contain a close capillary mesh. 

 The thinness and transparency are such as to allow of the 

 use of dry objectives as high as the ^V^h. In this case a film 

 of talc answers better than a glass cover. The tail is held 

 so still that the changes in progress in any given point where 



