154 The Microscope. 



crayons, No. 1 Lemercier's or Currier's, are used for drawing on the 

 grained papers, and can be mixed with pen work thereon. By 

 warming the back of a drawing made with lithographic crayons, 

 they are fixed more firmly to the paper and made blacker. Draw- 

 ings on enameled board are made more readily with a brush than a 

 pen. Solid blacks can be painted in sparingly with a camel's hair 

 brush. Pen lines run into these solids, infringing on the blacks, can 

 be picked up with the point of a sharp scraper and carried into the 

 solids, giving the effect of a wood engraving. This work can be 

 cross-lined with a brush, giving the effect of white stippling. All 

 drawings for process work should be pure black and white, even the 

 finest lines. Their color is best ascertained by using a magnifying 

 glass. 



The Examination of Watek. — Dr. Parkes, of London, after 

 reviewing the various methods of examining water, concludes that 

 chemcal analysis, aided by microscopic examination, is sufficient, in 

 the great majority of cases, to determine the amount of organic pol- 

 lution of a water, and whether it is of recent date. In many cases 

 the source of the pollution, whether from sewage or vegetable 

 matter chiefly, can also be determined, but there is no possibility of 

 ascertaining whether the water thus polluted is actually potent for 

 evil or whether it may not be entirely harmless. Chemical analysis 

 is powerless to deal with these cases of infinitesimal pollution of a 

 pure water with infective material from the human body. Cultivation 

 tests are equally powerless to cope with such cases. The only possible 

 way of ascertaining the probable eff'ects on the human system of 

 drinking such water, is for the operator to perform the experiment on 

 his own person — a course not likely to be pursued. The cultivation 

 tests, as now practiced, add very little to the results obtained by 

 chemical analysis. Micro-biology must undergo further development 

 before germ-cultivation methods can be expected to throw much 

 light on water pollutions. Lastly, the sanitary survey of the source 

 of the water, or its mode of storage, should always be carried out 

 whenever any doubt exists as to the freedom of the water from all 

 possible sources of contamination. — Med. News from Practitioner. 



Sense Organs of Tuebellakians. — Dr. L. Bohring has recently 

 published {Zool. Anzeigler, 260) a preliminaiy account of the eyes, 

 etc., of several species of planarians. In Planaria gonocephala each 

 eye consists of a nervous apparatus and a pigment -lay er, and the 

 latter, unlike the eyes of Lang's polyclada, consists of many uncleated 



