OBSERVING WITH AND MANAGING ENGISCOPE. 195 



that the body of the engiscope is truly Concentric with 

 the hole or diaphragm ; otherwise you will have no light. 

 Recollect, that the body of the instrument must now be 

 stationary , and that you must move the slip about with 

 your fingers. Whenever the diaphragms are employed, the 

 body must be a fixture , or nearly so. 



A very useful way of observing aquatic insects and 

 larvae is by means of the direct or diagonal boot drawn 

 over the lengthening piece, % of your object-glass, which 

 may then be inserted into the reservoir in which they 

 are kept. The method of mounting is as follows : Un- 

 screw the body from the arm, w, and attach fig. 15 or 16 

 to it, instead of its usual neck : see that the plane-glass at 

 their ends is as far removed from the object-glass as pos- 

 sible (for the less water you have to look through the more 

 distinct will be the vision) ; detach the stage from its pivot 

 by slackening the screw, /, and clamp it tight on the 

 lower pivot, , having previously removed the con- 

 denser, ft, altogether : then place the glass vessel on the 

 stage, which must be truly horizontal, and clamped tight 

 by the pinching screw, I : insert the body into the reser- 

 voir, by rescrewing it, with its new appendage, on to the 

 arm, w, again. If you want to observe laterally, or to 

 take a periscopic view of the contents of the vessel, use 

 the diagonal boot in place of the direct one ; and, by 

 turning it round on the lengthening piece, any object 

 situated on the sides of the vessel, such as polypi or vor- 

 ticellae, may be observed without disturbing them. The 

 object-glasses apply either to the ordinary neck of the 



