10 Mr. Sarjeant on Pond- Life. 



stopped out to suit the objective ; a good achromatic condenser 

 yields better results than the parabola, and with greater ease 

 and distance in working. It only requires the stop to be 

 changed or removed to adopt any other kind of illumination. 

 The spot lens, which is in reality a small bull's-eye condenser 

 with the central rays stopped out, is very effective, but an 

 ordinary stand bull's-eye condenser will give equally good results 

 if a disc of black paper about two-thirds (more or less according 

 to the angle of the objective) of the diameter of the lens is 

 pasted on the plane side ; or a very fair effect may be obtained 

 by simply using the oblique light reflected from the mirror when 

 pushed to one side, although by this method the image is very 

 apt to be blurred. 



With a quarter-inch objective, or one of higher powers, black- 

 ground illumination is out of the question ; and to get clear 

 definition with a delicate structure an achromatic condenser of 

 some kind is necessary. A good condenser is an expensive piece 

 of apparatus, but an ordinary microscopic objective reversed is 

 an excellent substitute, and it may be fitted very simply, either 

 under and attached to the stage, or to the substage. In using 

 an objective as a condenser, let it be of lower power than the 

 objective on the microscope ; thus with the quarter-inch objective 

 on the microscope the half-inch or two-third inch should be 

 adopted as the condenser. A small cap with a pin-hole in the 

 centre, dropped over the condensing objective, materially aids its 

 definition when employing direct light. 



In examining pond-life of any description the great point 

 towards successful illumination is to keep the object or objects 

 as much as possible in one plane by a cell as shallow as the size 

 of the organisms will permit. I have constructed a simple form 

 of compressor for this special purpose ; it has thin glass both 

 above and below, thus enabling it to be used with high powers 

 and with very oblique light, or to be reversed, should it be 

 required to examine both sides of an object. It is sometimes 

 rather a difficult matter to keep free-swimming rotifers, &c., in 

 the field ; but this may be generally overcome by fraying out a 

 very small portion of cotton-wool, placing it on an ordinary slip, 

 and dropping a little water containing the rotifers in the centre 

 of the wool, and covering it with an ordinary cover-glass ; the 

 network of woolly fibres prevents the rotifers from wandering 

 very far. Of course the amount of wool should be adapted to 

 the size of the objects, the main point being that it should 

 be just sufficient to restrain their movements without crushing 

 them. 



"When the construction of the microscope admits the use of 

 polarised light in conjunction with illumination on a black 

 ground, this is undoubtedly the best method of examining the 



