224 071 the Investigation of Mieroscojpic Forms. 



allied forms readily furnished the curve seen in Fig. 2, the small 



depressions being pits. 



This mode of experimenting often fur- 

 nishes us the means of determining whether 

 certain appearances are really due to open- 

 ings or to some other cause; thus the 

 small circles at the middle and ends of the 

 Pinnularia viridis have been mistaken by 

 some eminent observers for openings. Prof. 

 Bailey proved by the action of hydrofluoric acid that they are in 

 reality thicker portions of the shell, and examination by the method 

 here described shows that they are convex lenses, giving often very 

 well-defined images of the flame. The dots characterizing the 

 Coniferae furnish images of the flame indicating two or more 

 curvatures ; the ribs of the Pinnularia and the spaces between them 

 have opposite curvatures, &c., but the examples already given may 

 be sufficient to show the usefulness of the proposed method. 



Index of Befraciion of the Silica composing the Valves of the 



Diatoms. 



This point is closely connected with the foregoing, and it may 

 not be amiss to detail a few experiments that were made to deter- 

 mine it. 



Although Canada balsam has the same index of refraction as 

 quartz, still the valves of the diatoms which are composed of silica 

 are seen almost as distinctly in balsam as when mounted in water. 



To ascertain the relation between the index of refraction of 

 quartz and Canada balsam, independently of optical tables or labo- 

 rious experiment, I combined a convex quartz lens of 1 inch focus, 

 cut at right angles to the optic axis, with unheated fluid balsam 

 placed on a glass slide ; the two opposite refractions balanced with 

 each other so perfectly that the combination acted like a plate of 

 glass with plane parallel sides, and with ordinary means I was at a 

 loss to discover any tendency to convexity or concavity. Balsam 

 which had been heated was now combined with the quartz lens in 

 the same manner ; the balsam proved to have gained in refractive 

 power, so that the combination now acted distinctly as a concave 

 lens of weak curvature. 



Diatoms were then mounted in this unheated fluid balsam, in 

 which properly they should have been invisible, owing to the coin- 

 cidence of refractive indices, but, as had been anticipated, they ap- 

 peared beautifully, though perversely distinct. A casual remark 

 from Alex. S. Johnson, Esq., concerning a certain chemical difier- 

 ence he had often noticed between ordinary sihca and that com- 

 posing the diatom valve again turned my attention to this point. 



