428 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
imago of a grating became that of one with double the interval ; and 
the images of two gratings of which one had double the interval of the 
other could in this way be made precisely the same. The image of two 
gratings at right angles to each other could, by making use only of the 
spectra lying in a diagonal direction, be converted into that of a grating 
with direction of the strim along the diagonal of the gratings. 
Investigation of the Action of Nicol’s Polarizing Eye-piece.*— 
Dr. Sang in this paper, which is now printed for the first time, although 
read on February '2 Oth, 1837, explains the mode of action of the ordinary 
Nicol’s prism, showing how the ordinary ray is removed by total reflec- 
tion at the surface of the Canada balsam. He determines by mathe- 
matical calculation the correct inclination of the ends of the prism, and 
finds that the obliquity of the faces in the natural rhomb must be 
diminished 4° instead of increased as suggested by Nicol, the inventor, 
in his first description of the prism. A long and complex piece of 
analysis then follows to determine the best form of rhomb in order that 
the extraordinary ray may not suffer any deflection in its path, and that 
the extent of field may be as large as possible. The author concludes 
with the suggestion that a polarizer might be constructed of two prisms 
of highly refractive glass, separated by a thin plate of Iceland spar cut 
at right angles to the axis. 
In a note on the above paper, read November 23rd, 1891, Prof. Tait 
states how, at the urgent request of the late Dr. Sang, ho brought before 
the Council of the Royal Society of Edinburgh the question of its 
publication. Whatever judgment may be passed on the rest of the 
work, the paper contains a very important suggestion in the proposal to 
construct a polarizer of two glass prisms separated by a thin layer 
only of Iceland spar. With regard to Dr. Sang’s claim to priority 
in the explanation of the action of the prism, it is certain that many 
very similar attempts at explanation have been published since 1837, 
and the inventor himself supposed the action to be due to increase in 
the divergency of the two rays. 
In place of the complicated piece of analysis in Dr. Sang’s investiga- 
tion of the limits within which the prism acts, Prof. Tait gives a much 
simpler demonstration. The employment of glass prisms separated by 
a thin layer of Iceland spar has been recently suggested by M. E. 
Bertrand, f 
Schellback, H. — Der Weg eines Lichtstrahls durch eine Linse. (The Path of a 
Ray of Light through a Lens.) 
Zeitschr. Phys. u. Chem. Unterr., IV. (1891). 
Centralztg. f. Opt. u. Mech., XII. (1891) p. 97. 
(6) Miscellaneous. 
Dr. Van Heurck on the Microscope J — This book is intended to be 
a comprehensive treatise on the Microscope as constructed and used 
in its present form. In its treatment of the elementary principles of 
* Proc. Roy. Soe. Edinburgh, xviii. (1891) pp. 323-40. 
t Comptes Rendus, xeix. (1884) p. 538. 
i ‘ Le Microscope, sa construction, son maniement; la Technique microscopique 
en general; la Photomicrographie ; le Passe et 1’ Avenir du Microscope, par le 
Dr. Henri van Hcurck,’ 4th ed., Anvers, 1891, 8vo, 31G pp., text illust. 
