246 SUMMARY OF CURKENT KESEARCHES RELATING 10 



One line of the actual scale having been placed opposite one end of the 

 length to be measured, one of the lines of a diffraction image is made to 

 coincide with the other end ; the fractions of a division are read on 

 the circular scales. 



Modern Microscopy.* — This handbook of microscopy for beginners 

 and students, by M. I. Cross and M. J. Cole, has reached its fourtli 

 edition ; it has been revised and much enlarged, and now contains 

 chapters on special subjects by expert writers. Its general features are 

 so well known that no further allusion to their merits is needed. 

 Mention of the names of the writers of the special articles is suflBcient to 

 show that the subjects entrusted to them have been dealt with satis- 

 factorily. These subjects and their exponents are : 1. The Petro- 

 logical Microscope, by F. J. Cheshire. 2. Rotifera, C. F. Rousselet. 

 3. Fresh-water Mites, C. D, Soar. 4. Foraminifera, Arthur Earland. 

 5. Mosses and Liverworts, T. H. Russell. 6. Nature Study, W. M. 

 Webb. 7. Foods, C. Andrews. 



Determination of Refractive Index of a Liquid. t — The following 

 simple method of finding the refractive index of a liquid available in 

 small quantities is given by G. N. Pingriff. A plane 

 mirror A (fig. 38) is placed on the base of the stand, 

 and on it is put the double convex lens in such a posi- 

 tion that its centre is beneath the needle-point B. 

 With the eye directly above B, the observer adjusts 

 the sliding arm until the needle-point and its image 

 just coincide, as found by parallax. The distance 

 from B to the centre of the lens is then accurately 

 found — let it be/i. The experiment is then repeated, 

 after first placing a drop of the liquid upon the mirror, 

 when it will be spread out to a plano-convex lens 

 between the glass lens and the mirror — let the new 

 focal distance be/, ; then evidently the focal length/ 

 ; of the liquid lens will be given by 1/f = I//2 = I//1. 



; But since the focal length of the liquid lens is 



iP^ - A also given by the relation 1// = (/* - 1)/^, where r is 



'^ - _ "^ the radius of curvature of the surface of the glass lens, 

 it is evident that from a knowledge of r the index of 

 refraction of the liquid can be at once found. If r is not known it can be 

 found by putting a sheet of paper between the lens and mirror, and again 

 obtaining an image of B coincident with itself by reflection in the lower 

 surface of the lens. If this new distance from the lens is called d, we 

 have, since reflection is now only at the upper surface of the lens, 

 fx/r - 1/d = (jjL - 1)/ - r, or r = {2ix - 1)^, where /a now, of course, 

 refers to the glass, and can, if necessary, be calculated. The apparatus 

 is thus complete in itself, and three readings of the position of B give all 

 the data required. 



Weinbchenk, E.--Anleitung zum Gebrauch des Polarisations-mikroskops. 



Freiburg: Herder, Dritte Verb. Aufl., 164 pp. (167 figs.). 



* London: Bailliere, Tindal, and Cox, 1911, xviii. and 397 pp. (113 tigs.). 

 t Nature, Ixxxvii. (1911) p. 551. 



