316 THE AMERICAN MONTHLY [Oct. 



which it disappeared and did not return. In April, Coda- 

 nella lacustris appeared in this same pond, then it disap- 

 peared entirely, and was first found in other ponds in August. 



Objectives. — To many microscopists the terms "one 

 quarter inch," or "one half inch," as applied to their ob- 

 jectives, convey the idea that when in focus, the object is 

 at a distance of a quarter inch, or a half inch, from the front 

 lens. They confound the equivalent focal length of the ob- 

 jective with the working- distance. As a matter of fact, 

 the latter is always considerably less than the former. The 

 determination of the working distance of an objective is a 

 point of considerable importance, and therefore all micro- 

 scopists should make themselves familiar with the method 

 of calculating it. The following simple device will be found 

 useful for estimating the working distance of objectives 

 that are not higher than one-twelfth inch. Make a long- thin 

 wooden wedge, ten centimetres in length along the base, 

 and twenty millimetres in perpendicular height. Focus a 

 diatom on a glass slip without a glass-cover, and then care- 

 fully push the wedge along the glass slip until it touches 

 the objective. The thickness of the wedge at the point of 

 contact will represent the workingdistanceof the objective. 



Picro-carmine. — Prof. Leroy gives the results of his ex- 

 perience in the use of picro-carmine as a counter-stain for 

 bacteria in tissues. As a rule this reagent is somewhat 

 uncertain in its effects, and it is therefore suggested that, 

 to obviate risk of failure, the tissues should be first treat- 

 ed with logwood picro-carmine, and finally stained by 

 Gram's method of bacterial staining. As a counter-stain, 

 alum carmine alone gives only a nuclear stain and leaves 

 the cytoplasm practically untouched. Better results can 

 be obtained by first staining in alum carmine or borax car- 

 mine, then carrying the section through the regular Gram 

 process, and lastly leaving the section for half a minute in 

 a solution of sodium sulp-indigotate 0.1 gramme, and car- 

 bolic acid, five per cent aq. sol. one hundred cubic cen- 

 timetres, after which follow on with alcohol, creasote, and 

 balsam. By this method the nuclei will stain red, the cell 



