PHILOLOGY — HEMPIv. — PHYSICS — BARUS. 233 



(2) An article in the Transactions of the American Philological Associa- 

 tion, 1909. 



A preliminary report on some of his Cretan studies appeared in Harper's 

 Magazine for January 1911. 



The first report on his Etruscan studies has just been issued as an advance 

 reprint from the forthcoming Matzke Memorial volume, published by Leland 

 Stanford, jr. University. 



PHYSICS. 



Barus, Carl, Brown University, Providence, Rhode Island. Grant No. 697. 

 Continuation of study of the properties of condensation nuclei. (For 

 previous reports see Year Books Nos. 4, 5, 7-9.) $500 



In the forthcoming theoretical account of elliptical interferences, the author 

 has shown the practical advantages attained by associating the interferences 

 of thin plates with the diffractions of a transparent grating — a subject origi- 

 nally suggested to him by the phenomenon of coronas in which a marked 

 interference phenomenon is also superimposed on the diffractions. These 

 elliptic fringes may, however, be evoked in other ways than those discussed 

 in the report specified, and a number of these were investigated during the 

 present year. 



For instance, let the oblique mirror of Michelson's apparatus be the usual 

 plate of glass and replace the two opaque mirrors (usually called M and A'^) 

 by identical small reflecting gratings set at the angle of diffraction of the 

 spectrum, symmetrically to the incident rays. Here the elliptic interferences 

 will be seen in the telescope at right angles to the rays issuing from the 

 collimator. This adjustment is an inversion of the plan of returning the 

 diffracted spectra normally to the oblique transparent grating and the inter- 

 pretation m.ay be made along the same lines. The rings, however, are more 

 liable to be distorted, depending on the adjustment and ruling of the halved 

 gratings. 



Again, in a simple spectrometer adjustment for grating spectra (either by 

 transmission or by reflection), suppose the grating to be separated into two 

 halves by a division parallel to the ruling; then on displacing one of the 

 halves, micrometrically, parallel to itself from its original coplanar position, 

 interference shows itself in a way which is perhaps the most direct of any of 

 the methods hitherto treated. These fringes are rigorously straight, parallel 

 to the slit, and their size depends on the distance apart of halves of the grat- 

 ing, both parallel and normal to its original plane. They vanish after reach- 

 ing maximum size, but do not pass through infinity when the halves are 

 merely coplanar. The results have been treated in detail and they have a 

 bearing on the colors of coronas, inasmuch as spectra from sources in parallel 

 planes at a short distance apart along their normals are brought to interfere. 



