522 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
in concentric arcs are formed, having the violet inside and their con- 
cavity in the same direction as the annular layers. 
The author describes the eriometer , or instrument devised by Young 
in order to compare the dimensions of organic powders. It depends 
upon the phenomenon of crowns, consisting of a zone of concentric 
coloured rings, exhibited when a luminous point is examined across a 
glass plate on which a fine powder is spread. 
The instrument is formed of a divided scale C (fig. 54), to which are 
Fig. 54. 
attached three vertical plates. On the plate A the powder to be examined 
is placed. The eye is applied to a small aperture O in the fixed screen D. 
At B is a screen carrying one or more circular series of holes, and a 
wide slit across which a part of all the rings formed by the powder is 
seen, when a luminous point is placed behind the plate A. The screen JB 
is made to slide on the rule C in such a way that a ring of the same order 
coincides in all cases with one series of holes. The angular diameters of 
the rings are then between them as the distances of the screen B from 
the aperture 0. 
Upon the normal phenomena of spectra or of crowns produced by 
thin sections of wood are sometimes superposed phenomena of inter- 
ference which complicate them. In the case, for example, when there 
is a great difference between the dimensions of the clear spaces and 
the opaque v r alls, interference fringes are superposed on the diffraction 
fringes; but, as these interference fringes are very brilliant, they are 
easily distinguished. 
C6)] Miscellaneous. 
Introduction to the Study of Microscopy.* — Dr. E. Giltay’s work 
is intended as an introduction to the study of microscopy, and the 
intention is carried out in a logical and systematic manner by means of 
a consideration of seven objects, the microscopical characters and 
peculiarities of each being described and explained, such as coloured 
objects, starch-grains, milk, air-bubbles, diffraction plate. The descrip- 
tion of the objects is preceded by an introductory chapter explanatory 
of the parts of a compound Microscope. 
Micro-Chemistry.'f — Prof. H. Behrens’ manual is the first general 
treatise on micro-chemistry which has appeared in the English language. 
* * Sieben Objecte unter dem Mikroskop,’ Leiden, 1893, 66 pp. and 8 pis. 
f ‘ A Manual of Micro-Chemical Analysis.’ By Prof. H. Behrens. With an 
introductory chapter by Prof. John W. Judd. London, Macmillan & Co., 1894. 
See Nature, 1. (1894) pp. 122-3. 
