189I.] NEW-YORK MICROSCOPICAL SOCIETY. 79 



will scintillate so as to defy all efforts to define its outline. 

 Reflected innages look like actual details of structure in the ob- 

 ject. Dealing, as we constantly are, with objects made of glass, we 

 have constant use for our reasoning faculties to determine the 

 meaning of all these refractions and reflections, which sometimes 

 are almost as confusing as the broken images seen through the 

 glass pendants of a chandelier. 



In addition to these familiar effects of refraction and reflection, 

 we have the class of phenomena which we call diffraction effects. 

 These may be wave-like fringes of light and shadow following 

 the outline of the transparent object and reduplicating this out- 

 line ; or they may be analogous fringes thrown off the sub ivided 

 parts of the object, as from the cup-like outline of alveoli, or from 

 some projecting rib or groove like those along the diatom's 

 median line. 



We know by constant experience that when we throw light 

 obliquely through a transparent reticulated object like a diatom- 

 shell, the diffraction fringes from the separate alveoli run to- 

 gether across the shell in dark striae oblique or at right angles to 

 the direction of the light. In the Pleurosigma, in which the rows 

 of alveoli are oblique to the midrib, we very easily get the oblique 

 striation by the use of oblique light ; getting both series of lines 

 at once, one only, or one strong and the other faint, as we please 

 and with very little trouble. We get, with a little more pains, a 

 transverse striation, at right angles to the midrib, which is fainter 

 because it proceeds from alveoli not so closely connected in rows. 

 It may be called a secondary striation. With still more effort we 

 may get a much finer and fainter striation, parallel to the midrib, 

 by throwing light at right angles to it or nearly so. By lamplight, 

 and with objectives not apochromatic and not exceeding the 

 aperture of i.o N. A., these lines are usually in patches, upon 

 spots here and there, longer (in the length of the shell) than they 

 ■are wide. But with sunlight this tertiary diffraction striation may 

 be made to cover the whole surface of Pleurosigma angulatiim by 

 an exquisitely fine longitudinal grating over its whole surface, as 

 was demonstrated by Dr. Woodward in one of the most striking 

 of his photomicrographs in what is called " the Abbe experi- 

 ment."* As the improvement in our lenses, both by increasing 



* See R. M. S. Journal, vol. ii. (1879), p. 675; see also M. M. J. xvii., p. 82. 



