312 THE AMERICAN MONTHLY [Oct. 



paint, etc., becomes hard by oxidation, and not, as is gen- 

 erally supposed, by evaporation. The drying- process is 

 frequently a long - one. Where time is an object, as in class 

 demonstration, it may be expedited by placing- the slides 

 in a small chamber (a porcelain dish answers admirably) 

 and passing over them a stream of oxygen obtained by the 

 decomposition of potassium chlorate. 



Killing Objects. — A saturated solution of bichloride of 

 mercury, to which has been added one per cent acetic is an 

 excellent medium for killing specimens of the protozoa on 

 the slide or cover preparatory to mounting. In some cases 

 a hot solution is useful, as being more rapid in its results, 

 and producing little or no deformity of the cells. 



Dimensions. — It is a pity that amateur microscopists do 

 not more fully recognize the'necessity of recording,to some 

 uniform and convenient scale, the dimensions of the micro- 

 scopic objects that they make their study. This want of 

 uniformity not only detracts from the value of the work 

 done, but it also renders the work of comparison of the 

 drawings of the objects laborious and unreliable. By ope- 

 rating on some object of known size it is easy to ascertain 

 what arrangement of the microscope, and of its objectives 

 and ocularis necessary to obtain an image with the camera 

 lucida of any required size. Having determined on a given 

 amplification it should be adhered to, and a scale be made 

 corresponding to this amplification. The actual dimen- 

 sions of the object might then be readily ascertained by 

 applying this scale to the various parts of the image or 

 drawing-. The value of some such uniform method as 

 this is self evident. 



To Fix Objects. — To prevent sand grains and other loose 

 objects from being forced out from under the cover-glass 

 when mounting, Mr. H. C. Sorby recommends that the 

 objects should be well mixed with weak gum and water,the 

 gum to be of such a consistency as to make it easy to sep- 

 arate the grains and spread them uniformly over the space 

 which will afterwards be covered by the cover-glass. The 

 water is then allowed to evaporate slowly. Much of the 



