98 SUMMARY OF CURRENT RESEARCHES RELATING TO 



tube, D, with a lid perforated for the objective, completely surrounds 

 and incloses the object P. Through an aperture, 0, ordinary light con- 

 centrated by a lens can be concentrated on the object. The influence 

 of the yellow light upon such organisms as are sensitive to bright 

 ordinary light can then be studied. 



It is well known that Engelmann has shown, by arranging a spot of 

 light in a dark ground, that many organisms lose their activity when no 

 longer in the light spot. The author varies this experiment by intro- 

 ducing a light flicker in a yellow field, and his apparatus is shown in 

 figs. 10 and" 11. A simple small gelatin disk containing a small colourless 

 circle of 5 mm. diameter is projected into the plane of the preparation. 

 Racking of the illuminating apparatus will secure sharp definition of the 

 circumference of the circle in the image of the organisms. A cover- 

 glass acts as the glass disk, and the colourless circle is obtained by 

 application of a drop of hydrochloric acid and subsequent treatment by 

 a moist camel's-hair pencil. Any slight reddening due to the acid is 

 made good by ammonia. The cover-glass is fastened with strips of 

 paper pasted on to a large sheet of cardboard, whose plane is arranged 



normally to the incident light-rays. A 

 very slight displacement of the mirror 

 shifts the bright spot, so that organisms 

 which had previously clustered in the 

 white light come now under the in- 

 fluence of the yellow light. 



Dark-ground Illumination* — S. C. A. 

 describes how he successfully obtained 

 good dark-ground illumination with a 

 Zeiss DD X N.A.-85 one-sixth Reichert 

 7a N.A. • 87 one-seventh, using Baker's 

 Fig. 12. achromatic condenser N.A. 1*0. "Allthat 



is necessary is to insert a metal tube in 

 back of objective, one end of which comes in close contact with the back 

 lens of the objective. This end is split to enable a small metal disk to be 

 sprung into it ; the other end has a collar, which sits on the top of 

 objective. Two or three metal disks are necessary, each having the 

 centre punched out, giving varying apertures, a large or a small aper- 

 ture being used according to the amount of light to be stopped out. I 

 generally leave the tube in my J in., in which case all that is necessary 

 to obtain dark ground is to place expanding spot in condenser carrier. 

 This extra amount of work to be readily obtained from an objective 

 will be appreciated. Diaphragms, or a Davis shutter, are of no use for 

 dark ground illumination with high powers, as they work too far away 

 from back lens of objective." (fig. 12). 



Use of the Polariscope in Testing High-tension Insulators.f 

 C. F. Harding shows that with the aid of the polariscope it is not only 

 possible to determine some of the causes for the unsatisfactory service 



* English Mechanic, xc. (1909) p. 311 (1 fig.). 



t Proc. Indiana Acad. Sci., 1908 (issued 1909) pp. 147-9. 



