Observations on Ampliii^leura pellucida. By J. J. Woodward. 669 



of Amphipleura pellucida in balsam be examined successively by a 

 series of immersion objectives, the superior definition of those of 

 greater angle will be readily observed by the eye, and if photographs 

 are taken, will be equally manifest in them. 



In the series of photographs of Amphijyhura jpellucida that 

 accompany this paper, those numbered from 1 to 1 1 were all taken 

 with the angle of the illuminating pencil just mentioned, and will 

 serve to illustrate the accuracy of the statement made. The ex- 

 planation is quite obvious, for with sunhght the diffraction pencils 

 that radiate from the transparent spaces in the frustule are of suf- 

 ficient brilUancy to form a very conspicuous part of the image, 

 if brought to a focus by the objective ; and just what part of these 

 pencils shall actually be brought to a focus as part of the image, 

 depends in each case upon the aperture of the objective and not 

 upon the obliquity of the illuminating pencil. On the other hand, 

 by lamplight these difi'raction pencils are so much less brilliant that 

 they are inadequate to secure resolution, and it is necessary for this 

 purpose to use much more oblique illumination than will answer 

 the purpose by sunlight. 



In view of the foregomg considerations, it is not surprising that, 

 in the case of monochromatic sunlight, an increase of the obliquity 

 of the illuminating pencil produces no such marked improvement 

 in the performance of the objective as occurs under the same circum- 

 stances by lamphght. To illustrate this fact I have added the 

 photographs numbered 12 and 13, representing the same frustule, 

 as seen by means of an immersion illuminator, with the most oblique 

 light that could be used with each objective without distorting the 

 image. A comparison of these pictures with those taken by the 

 same objectives with the smaller angle of illumination, shows that 

 the improvement resulting from the increased obliquity is very 

 trifling indeed. These pictures will serve to illustrate the general 

 statement that with illumination by monochromatic sunlight, and 

 consequently for the purposes of photo-micrography, the superior 

 definition of objectives of greater balsam angle than 82'' is inde- 

 pendent of any excessive obliquity in the illuminating pencil, and 

 of the use of immersion illuminators. Indeed, as I shall take 

 occasion to illustrate hereafter by photographs, it is almost quite as 

 manifest with histological preparations and central light as with 

 lined test objects and oblique hght. 



III. Of the photographs which accompany this paper, Nos. 1 

 to 13 inclusive are representations of a frustule of Amphi'pleura 

 pellueida from a slide mounted in the Hull Botanical Gardens 

 in 1859. This slide was given to Mr. Wm. A. SuUivant, of 

 Columbus, Ohio, and by him presented to me in May, 1867. As it 

 came into my possession it was a dry mount with the frustules 

 adherent to the cover ; but for the purposes of these experiments I 



VOL. II. 2 Y 



