494 



SUMMAEY OF CURRENT RESEARCHES RELATING TO 



spectrum has a breadth b = d d u then the breadth with two-prism 

 spectrum will be 2 b sin <f>. 



Fig. 59 represents a spectroscope equipped with such a direct-vision 

 prism combination. The sleeve-collar of the one prism is provided with 

 an external drum graduated in degrees. 



Fig. 59. 



Tutton, A. E. H. — Das Elasmometer, ein neuer Interferenz-Elastizitatsapparat. 



[Full details and diagrams of this elaborate machine are given.] 



Zeit. f. Erystallogr. u. Miner., xxxix. (1904) p. 321. 

 See also Zeit. f. Instr., xxvi. (1906) pp. 163-7 (2 figs.). 



i(5) Microscopical Optics and Manipulation. 



Interferences produced by a Network limiting a thin Lamella.* 

 0. Meslin gives an explanation of the rings observed when a network 

 is placed on the convex surface of a lens of weak curvature, the strands 

 of the net being perpendicular to the plane of incidence. Such rings are 

 distinguished from Newton's rings by the following properties : — 



1. They are much wider and much further apart. 



2. They are visible in white light, even if network and lens are not 

 only not in contact, but several millimetres apart. 



3. They are scarcely iridescent, and when viewed at an angle of 45° 

 they are sensibly achromatic. 



4. The diameter of these circles diminishes when the incidence in- 

 creases : with Newton's rings the opposite effect holds. 



The author attributes the phenomenon to the interference of the 



* Comptes Rendus, cxlii. (1906) pp. 1039-42. 



