«86 THE MICROSCOPE. 



second paper of this series, excepting the use of the white zinc 

 cement, in place of balsam. The ground work or foundation ring 

 ot cement is to be made upon the glass slip with this material, and 

 allowed to harden thoroughly; then another layer is added and the 

 circle of wax laid upon it, pressed down and covered with at least 

 two coats of zinc, inside and outside. If not deep enough, a second 

 or even a third ring of the wax may be added, care being taken to 

 cover each completely with the cement. When thoroughly dry and 

 hard, the cell is to have a fresh coat of cement run upon its top sur- 

 face, be filled with the preservative fluid, and the object (previously 

 soaked in the same) is to be placed within it, any floating or hidden 

 air bubbles removed with the needle, and the cover placed on, excess 

 of fluid expelled, washed, cemented, etc., etc., precisely the saine as 

 we did in the case of the shallow cell. 



THE PODURA SCALE. 



BY A. V. MOORE, M. D. 



IN the December issue of the American Monthly Microscopical 

 Journal appears an article by the editor. Prof. R. Hitchcock, 

 upon the scales of that ametabolian articulate commonly known as 

 podura. 



In the article referred to, we are told that, although not dif- 

 ficult of resolution, the scales are valuable for testing objectives of 

 all powers; that eminent authorities have differed in their interpre- 

 tations of the appearance of the scales; that some objectives give 

 appearances differing entirely from others; and further, that it is not 

 now a question of structure but of appearance. 



Now, it may be that I misunderstand the import of this; but, if 

 not, it is a peculiar statement. 



If it be no longer a question of structure, why should it be a 

 question of appearance ? If the structure be known, the appearance 

 must accord with that structure, modified by the difference in the 

 color correction of different objectives. 



We are told that some objectives show the exclamation points 

 or spines with a longitudinal slit of light, tapering at both ends; 

 while other objectives show the line of light with a distinct head 

 from which the line tapers. 



Now, I fear that the gentlemen by whom the examinations were 

 made, did not give sufficient attention to the spherical correction; 

 for, with the spherical aberration properly corrected and the light 



