MICROSCOPICAL TECHNIQUE. 223 



words, the fact that fifty per cent, of the cutworms in his field are 

 infested by parasites does not help the farmer in the least." 



But obviously it does help the farmer very greatly the next 

 season, for it reduces by half the number of cutworms he will have 

 to contend with. As a matter of fact, cutworms fluctuate in 

 numbers in a way quite similar to the army worm, and the fluctua- 

 tions are largely due to parasitic enemies. I have seen regions 

 where cutworms were so abundant that grain-fields were literally 

 cut off by them as by a mowing machine, and the following season 

 the worms were so scarce as to do practically no damage. 



But Professor Smith is right in saying that as a general rule 

 there is too great a tendency to rely on natural enemies to subdue 

 insect attack. It is nearly always safer to adopt effective measures 

 in keeping pests in check than to trust to the chance of their 

 natural enemies subduing them. 



flD(cro6coptcal ^ecbnicjue. 



Compiled by W.H.B. 



Koch's Solution of Methylene Blue. 



Saturated alcoholic solution of Methylene Blue i drachm. 

 Solution of Caustic Potash, lo per cent. ... 12 minims. 



Distilled Water ... ... ... 200 drachms. 



The stain is very active in a feebly alkaline state. 



Artificial Reproduction of Anhydrite from Evaporation of 

 Salt Solutions. — Brauns * has produced anhydrite in microscopic 

 crystals by bringing upon an object-glass a large drop of a satu- 

 rated solution of sodium or potassium chloride, or a mixture of 

 the two salts, and placing to one side of this a drop of calcium 

 chloride solution, and on the other side a drop of Epsom salt 

 solution. The three drops are joined to one another by narrow 

 paths and evaporated. During the diffusion of the liquids which 

 takes place, calcium sulphate is formed and appears in crystals of 

 both gypsum and anhydrite along with the crystals of the chlorides. 

 When a little water is added to a group of anhydrite crystals, they 

 are dissolved to re-crystallise as gypsum. By properly regulating 



* Neues /ahrbuch f. Min., etc., 1894, II., pp. 256 — 264, in American 

 Naturalist^ Feb., 1894. 



