INDUSTRIAL PROGRESS DURING THE YEAR 1875. cxxxvii 



different media (balsam, glass, and water), and the angle at 

 which it enters the objective must therefore be greater than 

 41. Id examining Moller's Probe -Platte, a balsam mount, 

 under these conditions, with light from a kerosene hand- 

 lamp, I easily resolved the Amphipleura pellucida ; so clear 

 and decided were the lines that with a power of 8000 they 

 were still visible. 



" The resolution of this difficult diatom, as well as the 

 Frustulia Saxonica and Nitzschia curvula (Nos. 18 and 19 

 on the Probe-Platte), far surpasses any that I have ever 

 seen by artificial light, and rivals the beautiful resolution 

 obtained by monochromatic sunlight. With this illumina- 

 tor it is much easier to resolve the Amphipleura in balsam 

 than to resolve it dry with any other artificial illumination. 

 The advantages of the reflex illuminator in thus furnishing 

 light of greater obliquity than has been obtained by other 

 methods seem to me worth considering by those interested 

 in testing the resolving power of objectives. 



"It is advantageous to connect the illuminator with the 

 slide by glycerin, instead of water, as it does not evaporate. 

 The higher refractive power of glycerin makes no difference 

 in the ultimate direction of the light. 



" With high amplification the lines of the Amphipleura 

 become decidedly beaded, but do not separate into dots." 



We commend to the careful reading of microscopists and 

 microscope - makers the excellent paper of Mr. Slack, read 

 before the Royal Microscopical Society of London, May 5, 

 1875, and the discussion thereon, contained in the June num- 

 ber of the Monthly Microscopical Journal. The paper is 

 entitled, "On Angle of Aperture in Relation to Surface 

 Markings and Accurate Vision." Mr. Slack proves, from the 

 results already accomplished by Zeiss, of Jena, working un- 

 der the direction of Professor Abbe, that resolving power 

 and penetration are not in that condition of irreconcilable 

 hostility generally supposed, and that a new era is dawning 

 upon physiologists, and, indeed, all who care for something 

 more than the mere display of diatom dots. It is well 

 known that in the extravagant desire to display these dots 

 angular aperture has been pushed to an extreme, and a cer- 

 tain amount of chromatic error allowed as necessary to 

 sharpest definition. By very careful construction, centring, 



