672 Transactions of the Socieitj. 



objectives capable of resolving this test has been constantly in- 

 creasing, and the sharpness of the images produced by the very 

 best objectives has continually improved, until at the present day 

 I have no hesitation in making the assertion that any first-rate 

 immersion objective, even those of as low power as the i, ought to 

 be able to give distinct resolution of the most finely marked 

 frustules of Am])liifleura ijellucida mounted in Canada balsam. 

 Those incapable of this performance must be classed among second- 

 rate objectives, and will not willingly be employed in serious in- 

 vestigations by instructed microscopists. As for the appearance of 

 balsam-mounted frustules of this diatom by sunlight, under the best 

 modern immersion objectives, it not only rivals in vigour and 

 contrast the finest views of the dry frustules, but greatly excels the 

 best that can be done with these in simultaneous exhibition of the 

 details of outline and midrib. 



The frustule on the Hull slide selected for photographic repre- 

 sentation in connection with this paper is '0037 of an inch long, 

 and has 102 striae to the thousandth of an inch. At the distance 

 of •0011 of an inch from it, and nearly parallel to it, on the slide 

 is a second rather more coarsely marked frustule, the appearance of 

 ■which in the pictures will aid in forming a judgment of the flatness 

 of field in each case. Near one extremity of the selected frustule 

 appears the end of another that may be used for the same purpose, 

 while near its other extremity the end of yet another frustule is 

 seen, at right angles to it, which, from its position in relation 

 to the light, is not resolved, although longitudinal diifraction lines 

 appear upon its surface in all the pictures. 



All the immersion objectives belonging to the collection of the 

 Museum will show the striae on the selected frustule by monochro- 

 matic sunlight ; but some of them, from curvature of field and 

 feebleness of definition, would have given but sorry pictures. I 

 have only thought it worth while to make photographs with a few 

 of the very best of the water-immersion objectives at my disposal, 

 and with certain glycerine and oil-immersion objectives. The 

 following is a list of the photographs, to which I have added a 

 memorandum of the aperture of each objective, as measured with 

 an instrument I have devised, a modification of the apertometer of 

 Abbe, which I will describe fully in a separate paper. The angles 

 are all computed for an ideal medium of 1 • 5 index of refraction. 



(a) Photographs of Amphipleura PELLUCIDA, illuminated by monochromatic sun- 

 light. Condenser a 3-inch objective of 12° air aperture inclined at an angle of 45° 

 to the optical axis of the Microscope. 



No. 1. Oil-immersion -^\, aperture 114°, by Zeiss, vignetted print. 2830 

 diameters. 



No. 2. By the sunie olijcctive; print not vignetted. 2760 diameters. 

 No. 3. Oil-immerbion J, apeiture 115°, by Zeiss. 2700 diameters. 

 No. 4. Oil-immer.sion ^, aperture 122°, by Tolles. 2700 diameters. 



