On ilie Structure of Diatoms. 19 



My test measurements of angle were made according to this 

 method. An exact semi-cyHnder in use, carefully in proper posi- 

 tion, and an object mounted in balsam under covering glass imme- 

 diately on the plane surface of the semi-cylinder itself. It was 

 used where a balsam-angle of 110° was shown, the method de- 

 scribed, and with the result published.* The method has not 

 been," and will not be, impeached. The results were verified by 

 putting the light (sunlight) down through the tube. 



When the maximum angle limits have been ascertained, then 

 the angle can be measured in the way Mr. Wenham points out, 

 viz. by rotating the microscope. Or, if the semi-cylinder be pro- 

 vided with divisions to degrees, adjacent, a shutter sliding over the 

 cylindrical surface from the central parts outward will, under 

 observation at the eye-piece, mark exactly the limits of angle on 

 either side, and the corresponding degree can be noted. According 

 to Mr. Wenham, only 82° can be seen there with an object-glass 

 properly so called, with air or water or balsam contact at plane 

 surfaces. Now, note the fact that in my hands in June last j this 

 occurred — viz. when 81° closely was shown on the screen as the 

 aperture of an objective " dry," then, by simply making the 

 medium of contact at the plane surfaces water, or balsam, instead 

 of air, the pencil-limits on the screen were found at about 110°, 

 the object in focus in both instances. 



And then, to show that this additional pencil consisted of 

 " image-forming rays," the moist-paper screen was removed, and a 

 shutter, covering 82°, substituted, and on illuminating the object 

 through this outside portion only, behold, with the objective at 

 maximum angle adjustment and used dry, total darkness was the 

 effect of course to the eye at the eye- piece, while if balsam or 

 water contact was given, the light had access beyond 82°, and the 

 object became in clear view well defined. 



Thus fact and " theory " continue (!) consistent. 

 Jacksonville, Florida, May 14, 1874. 



IV. — On the Structure of Diatoms. By G. W. Moeehouse, U.S.A. 



It is hoped that the publication of the following memoranda will 

 serve the double purpose of elucidating the structure of the tests, 

 and at the same time demonstrating the utility of microscopical 

 objectives of exceptionally high powers. The uncertainty of the 

 footing in this unstable and contested ground will necessitate many 

 errors, and may serve as an excuse for them. So many competent 



* See ' ]M. M. J.' for June and August, 1873. 

 t See ' M. M. J.' for August, 1873. 



