234 THE AMERICAN MONTHLY [Aug. 



ciliary phenomena occuring alternately at both ends or 

 prows of the Navicula species will not be so evident 

 when studied under a 1-6 lens. For the penetrating 

 power of the lens is very much reduced, when compared 

 to the depth of layers of granules that the 1-2 inch lens 

 will accommodate. For the appearance of granules of 

 carbon rushing forward to contact with the frustule, or 

 their apparent repulsion from the frustule, the 1-2 inch 

 power is best adapted in order to demonstrate these 

 various phases. The diatoms will appear relatively 

 small, but the phenomena distinct, by using higher eye- 

 pieces without changing the 1-2 inch lens. The effects 

 may be followed up to about 600 magnification. 



But to witness the more interesting phases that com- 

 pel a recognition of the evident Protozoan character of 

 the NaviculsB, it is indispensible to use the highest 

 powers possible. All the critical results of importance 

 arrived at by myself were secured by the use of a Zeiss 

 D lens, (equal to l-6th,) A and B eyepieces, and a 1-2 

 inch Beck & Smith lens reversed in use, and adapted as 

 an eyepiece. The latter combination resolved the test 

 lines on Grammataphora marina, and some twenty-five 

 or more species of Pleurosigma on a type plate of the 

 Bay of Naples forms. The change from one eyepiece to 

 another can be made at any opportune moment with- 

 out loss of time, or changing the objective. 



In regard to the lighting, and some other requisites of 

 manipulation, an Argand burner lamp is used, a bull's- 

 eye condenser being adjusted as near as possible to the 

 flame, and a large image of the flame projected so as to 

 fall on the concave face of the mirror. To the substage 

 an achromatic condenser is adapted, and when the light 

 is properly centered in the field, the result will be a 

 dazzling light. But in order to guarantee the success- 

 ful view of the various phenomena, it is necessary to 

 have at hand a glass slip, or smaller piece of emerald or 



