ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 253 



chequered web formed of two systems of parallel and opaque equidistant 

 bands, whose width equals that of the transparent intervals, the bands 

 intersecting at right angles and so forming square transparent meshes. 

 If such a web is placed a short distance in front of a sensitive photo- 

 graphic plate inside a suitably constructed apparatus, the diaphragm in 

 the objective projects behind the web cones of full light, and cones of 

 pure shadow connected by a zone of varying penumbra. It is possible 

 to determine the variations in the illumination of the sensitive plate 

 when the diaphragm is subject to ordinary conditions, that is when the 

 diaphragm presents a square aperture whose diagonals are parallel to the 

 hands of the web, the dimensions being such that the plane of the 

 sensitive plate contains the vertices of the cones. This will be the case 

 when the ratio of the diagonal of the mesh to the distance between the 

 web and the plate equals the ratio of the side of the diaphragm to the 

 extension of the camera. In the absence of the web and after focusing, 

 the diaphragm seen from any point of the web-plane appears to be a 

 luminous surface of uniform brightness, the said brightness being 

 proportional to that of the point conjugate to the object-point at which 

 we supposed the eye placed. The light-beam illuminating each point tt 

 is limited by the pyramid having tt for vertex and the contour of the 

 diaphragm for base. Owing to the interposition of the web, there will 

 be a reduction of the illumination at tt if the opaque bands of the well 

 which impinge on this pyramid, reduce the useful section of the beam 

 by the plane of the web. This section can be taken as a measure of 

 the illumination. The authors show that the curves of equal illumination 

 (isophotic curves) take the forms of circles and of equilateral hyperbolas. 

 They also show that a knowledge of these curves renders possible a 

 previous determination of the web-image at a given part of the sensitive 

 plate. 



Advances in Photomicrography and Projection.* — In the Jahr- 

 buch fiir Photographie und Reproduktionstechnik fur das Jahr 1908, 

 G-. Marktanner-Turneretscher summarises, with his usual thoroughness, 

 the most important contributions to photomicrography and projection, 

 published in the scientific journals of different countries. The results 

 are classified under the headings of : (a) Mikrophotographie ; (b) Pro- 

 jektion ; (c) Kinematographie. 



Jencic, A.— Ein wichtiger Fortschritt der mikroskopischen Beleuchtimgmetlioden. 

 Allgm. Zeitschr. f. Bierbr. u. Malzfabrikat., xxxvi. (1908) pp. 179-82. 



(6 figs.) 



(5) Microscopical'Optics and Manipulation. 



Brownian Movements.!— J. Perrin,in commencing his article on the 

 above subject, reminds his readers that every particle situated in a fluid 

 in equilibrium, is in a condition of continuous and quite irregular agita- 

 tion in proportion to its minuteness. Gouy has shown that this eternal 

 agitation is an essential property of fluids, and has suggested a very 

 attractive explanation, by supposing' that it is a visible consequence of 



* Edited by Dr. J. M. Eder. Halle : W. Knapp ; also separately as a pamphlet. 

 + Comptes Rendus, cxlvi. (1908) pp. 976-70. 



