180 The Microscope. 



ter. Glass plates and a linen prover have been made use of, but 

 for more accurate results other and better means are called for. 

 The Esmarch tube is simply a test tube evenly coated internally 

 with solid, sterilized nutrient medium — agar-agar, gelatin, combi- 

 nations of the two, etc., — and stopped with cotton. The coating 

 is done by pouring into the tube a quantity of medium, tipping 

 and turning the same until no part of the surface remains un- 

 touched, except of course that in the immediate vicinity of the 

 cotton stopper. When the medium has been thus evenly spread, 

 the tube is immersed to the neck in ice water, and then stored 

 for future use. Some roll the tubes on ice, but the medium 

 hardens and sets unevenly — in lumps, ridges, etc. — a condition 

 of things likely to vitiate the count. 



In making a comparative determination a series of tubes are 

 taken, a given quantity of the material under examination put 

 into each one, " swashed " about and the surplus thrown out, or- 

 by means of gentle heat (not however always advisable), incor- 

 corporated with the medium. At the end of a given number of 

 hours or days, a count is made, the count repeated at intervals, 

 the results recorded, and if it is desired to experiment further a 

 cultivation begun. 



At this stage of the examination the counter comes into play.. 

 It is simply a small microscope adapted to tube examinations 

 and consists of a modification of a brass knife-clamp that grasps 

 the tube, holding it firmly to the under side of the stage, the 

 opening in which contains a cover-glass divided into square 

 millimetres, or in a more recent and better form, an opening in 

 the stage one by four millimetres, the greater diameter running 

 lengthwise with the tube. The optical part is an " Excelsior" 

 triplet, the lenses of which may be used separately or in combi- 

 nation ; the adjustment is frictional. The substage has universal 

 movements, and may be readily detached if window or lamp- 

 light is preferred direct. The Bausch and Lomb Optical Com- 

 pany make the instrument. 



