lO AQUATIC MICROSCOPY FOR BEGINNERS. 



The arm and the foot may be made of either brass 

 or iron, but there should always be a joint between 

 them so that the upper part of the instrument may be 

 inclined. The cheapest stands are made without this 

 arrangement, and they must therefore a' ways be used 

 in a vertical position, the observer being compelled to 

 hold his head and body in a way that soon becomes 

 exceedingly wearisome. An iron arm and foot are 

 quite as useful as if made of brass, but no stand 

 should be selected without the joint for inclination. 

 Brass looks better, and is more expensive than 

 neatly japanned iron, but is practically no more useful. 



The microscope-body should be about ten inches 

 long. In the less expensive stands it is often made 

 in two parts, the upper tube sliding within the other, 

 so that when drawn out to its full extent the entire 

 body will be of the proper length to obtain the best 

 results from the objectives. In fuch a stand, when 

 the inner tube, or "draw- tube" is pushed down, the 

 microscope will have the lowest magnifying power 

 obtainable with the eye-piece and the objective then 

 in use; when fully extended, the power of the objective 

 will be greatly increased, so that by varying the 

 length of the body by the use of the "draw-tube," 

 many different magnifying powers may be obtained 

 from the same low-power objective. In some cases this 

 arrangement may be useful; it is at least not entirely 

 objectionable, neither is it entirely convenient. 

 Stands with an undivided body ten inches long — the 

 standard length — also often have d secondary draw- 

 tube by means of which the body can be enormously 

 lengthened, and the magnifying power enormously 

 increased, but usually with a loss of s'ome good 



