112 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



(5) Microscopical Optics and Manipulation. 



Lyman, T. — The Prolongation of Spectral Lines. 



[Explains the cause of the streamers observe! in the use of a concave grating.] 

 Proc. Amer. Acad, of Arts and Sciences, xxxix. No. 2 (July 1903) 



pp. 33-5(1 pi.). 



„ „ Explanation of False Spectra from Diffraction Gratings. 



[Shows that they seem due to a so-called periodic error in the gnuing ruling.] 

 Proc. Amer. Acad, of Arts and Science*, xxxix. No. 3 (July 1903) 



pp. 39-47 (1 pi.). 



(6) Miscellaneous. 



General Principle of some Novel Forms of Geodetical Instru- 

 ments.* — Sir H. Grubb describes an important simplification in geo- 



= = ^ 



Fir. 15. 



detical instruments, which may also be found useful in some departments 

 of microscopy. The half-silvered, half-plain piece of glass generally 

 used in such instruments is replaced by a piece of glass having a thin 

 film of lead sulphide deposited on its surface. This film both reflects 

 and transmits the incident light, and by varying its thickness the pro- 

 portion of transmitted to reflected light may be varied. The effect is 

 that the images of two objects may be got actually superposed, instead 

 of (as in the prismatic compass) vertically above and below one another. 

 The arrangement will be understood from fig. 15, where the distant 

 object is seen by direct vision through the film, and the near object 

 (after collimation) by reflection. One great advantage is that all the 



* Brit. Opt. Joum., iii. (Oct. 1903) pp. 29-31 (4 figs.). 

 (Oct. 30, 1903) pp. 263-4. 



Eng. Mech., lxxviii. 



