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, 'i, 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, iv, 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, I, and clamp it tight on the 

 lower pivot, i, having previously removed the con- 

 denser, n, 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, iv, 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- 

 ticelhe, may be observed without disturbing them. The 

 object-glasses apply either to the ordinary neck of the 



