Neiv Apertometer. 279 



mounted in Canada balsam. It is a finely marked specimen with 

 102 strire to the thousandth of an inch. The magnifying powers 

 were intended to be made approximately equal, but in fact vary 

 from 2700 to 2830 diameters. I have also added a photograph of 

 the same frustule by a Spencer glycerin immersion \ inch, magni- 

 fied 1900 diameters, and an enlargement from this negative to 

 2760 diameters. The angle of this objective was not determined 

 in my new apertometer, but measured in another way 106", in a 

 medium of 1.5 index. The pictures, numbered from i to 11, were 

 all taken with the illuminating pencil inclined but 45° to the 

 optical axis of the microscope, and without any immersion sub- 

 stage contrivance. They open a number of questions which I will 

 discuss elsewhere. I will only call attention here to the fact that 

 while, in a general way, they show the superiority of glycerin and 

 oil over water immersion objectives, the great superiority of the 

 performance of the glycerin objectives over water objectives of 

 very nearly the same aperture, indicates that other advantages are 

 possessed by the denser immersion media besides the mere pos- 

 sibility of increased angle. The' nature of these advantages has 

 been correctly discussed by Professor Abbe (see Journal of the 

 Royal Microscopical Society, May, 1879, p. 256), in the case of the 

 oil objectives, and the glycerin objectives share them, though not 

 to the same degree. 



I have added to the series two photographs, numbered 12 and 13, 

 taken by the Zeiss oil -^ inch and the Tolles oil yV inch, with the 

 greatest obliquity in the illuminating pencil that could be given by 

 means of an immersion illuminator, and in each case without 

 distorting the image. A comparison of these pictures with those 

 of the same frustule, taken with the saiiie objectives and with the 

 obliquity used in the other pictures, will serve to indicate that with 

 monochromatic sunlight the greatest possible obliquity in the illu- 

 minating pencil is, by no means, essential to secure the advantages 

 resulting from excessive aperture. I will elsewhere take occasion 

 to discuss the meaning of this phenomenon and its relations to 

 lamplight illumination. Finally, I send another photograph of an 

 exceedingly delicate frustule of Amphipleura pellucida, mounted 

 dry, by the Zeiss -^ inch, to illustrate the superb performance of 

 objectives of great angle on dry objects also, provided these are 

 adherent to the covering glass. This frustule has 105 striae to 

 the thousandth of an inch, and is magnified 3400 diameters. 



