ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



947 



Fig. 224. 



position beneath a thread of soft cotton which is connected with a 

 vessel of water, the water by capillary attraction and gravity runs 

 down the cotton and falls drop by drop upon the centre of the cover, 

 and quickly makes its way 

 through the hole to the 

 under side. When the space 

 beneath the cover is full 

 the water will then have 

 reached the thi-eads at the 

 edge, and will so be drawn 

 off. There is thus a con- 

 stant flow of water supplied 

 by the thread above and 

 carried off by those at the 

 edge of the slide. 



Objects may be intro- 

 duced into the slide through 

 the hole in the cover, being 

 first taken up in the dipping 

 tube and allowed to flow 

 slowly through the hole, and 

 so under the cover. 



A number of slides may be in operation at one time, it being 

 possible to arrange as many as ten or a dozen round a base from 

 which threads convey the water to each slide. 



The slide was described by Mr. T. Charters White (at the last meet- 

 ing of the Society *) as the best form which he had yet worked with. 



Diaphragms for Axial Condensers. — At p. 667 it was stated 

 that " no disposition of diaphragms has yet been applied to condensers 

 used in the axial substage to enable us to regu- 

 late the amount of light without altering the 

 ohliquity." 



With the simple diaphragms, however, shown 

 in Fig. 225 (which were, we think, originally 

 used by the Eev. J. B. Eeade, with his " kettle- 

 drum" illuminator) the obliquity of the light 

 is unaltered when the upper diaphragm is rotated 

 over the lower, or vice versa (the lower indicated 

 by dotted lines), while the amount is regulated 

 axially at pleasure, from the maximum when 

 the two slots coincide to an almost infinitesimal pencil when they are 

 separated. 



New Dioptrical Formula. | — The following is published by the 

 French Academy on the report of Messrs. Fizeau, Jamin, and Cornu. 

 The authority is Mr. C. V. Zenger, 



" It is acknowledged that the abridged dioptric formulae do not 

 furnish results sufficiently exact for them to be employed in the con- 

 struction of aplanatic and achromatic objectives. 



* See post, p. 979. t Comptes Kendus, xciii. (1881) pp. 398-9. 



3 R 2 



Fig. 225. 



