The Microscope. 305 



considered a Pleurosigma. But that does not matter. What I 

 want to ask is, How do the Diatoms see ? They have no eyes 

 or nerves in fact. Are they animals or vegetables ? I watched 

 these for some time, and this I saw : A Gollefonema eximium 

 had a tube broken off in such a manner that it could come out, 

 which it did. It came trembling out until it was all out but 

 about one-eighth. Then it went back again. This it repeated 

 several times. At last it came out entirely, until it was four or 

 five times its length away from the tube. It pointed crosswise to 

 the tube, and I thought it would sail away. But soon it turned 

 about, and without feeling for the entrance of the tube pushed 

 its way back, and seemed to wait as if it were tired. After a 

 time it again came out, and sailed away six or eight times its 

 length, and then it went back into the tube and remained there. 

 Now how could it see its way into the tube without eyes ? It 

 sailed straight back without feeling around. When it came out 

 it stretched the tube, seeming to push its way until it was nearly 

 out, and then, with a sudden bound, it squeezed past the opening 

 and shot its way into the world. This I saw more than twice. 



NOTES ON THE MICROSCOPE STAND AND ON SOME 

 OF ITS ACCESSORIES. 



AN AMATEUR. 



XXI. 



THIN GLASS — CONTINUED. 



ri^HE Jou7'nal of the Royal Microscopical Society reports from 

 -■- Dr S. Czapski the following method of determining the 

 thickness of cover-glasses of mounted preparations, a measure- 

 ment which it is often very important to know for high powers. 

 The procedure pre-supposes the possession of some cover 

 glasses, the thickness of which is known, and that tlie head of 

 the fine adjustment screw is divided by radial lines. 



The upper and lower surfaces are focussed with central 

 illumination, and the amount of turn given to the fine adjust- 

 ment screw noted for each cover glass, it being unimportant 

 whether the exact value of the screw turn is known or not. If 

 the surfaces of the cover glass do not present any obvious 

 marks to focus on, an artificial one, such as dust or scratches, 

 20 



