1893.] MICROSCOPICAL JOURNAL. 113 



water effectually and economically by a combination of heat and 

 pressure. As described in the Joum. de Microgr , xvi, pp. 

 145-^3, it consists of four distinct parts — a boiler, primary and 

 secondary converter (or cooler) , and a clarifier. The water to 

 be sterilized is introduced into the primary converter — a cylindri- 

 cal metal vessel surrounded by a worm in which water heated to 

 i2o"-i30°, and just coming from the boiler, is circulating. From 

 the converter, and having been raised to 100°, the water is con- 

 ducted along a pipe to a worm running around the boiler, where 

 it is heated up to i3o''-i3o". From this worm the water then 

 passes through the worm in the primary converter, thence through 

 the secondary, converter, and finally, having passed through 

 the clarifier, completes its circuit. The secondary converter is 

 also a worm surrounded by cold water, and might be termed the 

 cooler. The clarifier is filled with powdered silica, apparently 

 between layers of canvas, and is not intended for a filter, but to 

 impart a clearness or limpidity to the water which has been re- 

 moved from it bv the heating it has gone through. The water 

 having passed through the clarifier is delivered bright and clear 

 and fit for all the purposes of life. 



To compute the magnification of an object photo- 

 graphed with or without an eyepiece and at different 

 distances from the objective. — The easiest way to find this 

 magnification is to remove the object after photographing it. 

 Put a stage micrometer in its place, throw the image of the 

 lines on the ground glass and compare them with an ordinary 

 rule. This will give the correct magnification produced by that 

 optical combination. To compute it more or less accurately, 

 reckon that an inch objective will amplif}' 10 diameters at 10 

 inches from its optical centre. If an inch eyepiece is used mul- 

 tiply ag;iin by 10. Other objectives, eyepieces, and distances 

 in proportion. 



Nelson's formula is . „ magnification, A = focal rating 



of eyepiece, B = focus of objective, O = optical tube length 

 (from optical centres), D distances from object to eye or 

 ground glass. As eyepieces and objectives are rarely rated cor- 

 rectly, computation by any^ rule will give approximate results 

 only. 



Killing Nematodes for the Microtome. — Mr. C. W. Stiles, 

 Atnerican Naturalist {i8g2) recommends the following method : 

 Only one worm can be killed at a time ; place it on a large 

 slide with a few drops of water ; place a second slide over the 

 worm and move it slowly to and fro. This movement causes 

 the worm to straighten. As soon as the Nematode assumes tlie 

 desired position pipette in the fixing solution between the slides, 

 continuing the motion of the upper slide until the worm is dead. 

 By this method a specimen can be obtained which is perfectly 

 straight and sound. Pressure on the delicate worm may be 



