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 Aug-ust. 

 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 leng-th 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 hig-her than one-twelfth inch. Make a long thin 

 wooden wedg-e, ten centimetres in leng-th along- the base, 

 and twenty millimetres in perpendicular heig-ht. Focus a 

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

 fully push the wedg-e along the g-lass slip until it touches 

 the objective. The thickness of the wedg-e at the point of 

 contact will represent the working- distance of the objective. 



Picro-carmine.^ — Prof. Leroy g-ives the results of his ex- 

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

 bacteria in tissues. As a rule this reag-ent is somewhat 

 uncertain in its effects, and it is therefore sug-g-ested that, 

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

 ed with log-wood picro-carmine, and finally stained by 

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

 alum carmine alone g-ives 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 reg-ular Gram 

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

 a solution of sodium sulp-indig-otate 0.1 g-ramme, 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 



