NUTTING AND JONES: PHOTOMETER FOR SMALL AREAS 313 



focal plane of the positive eye piece, with an accuracy nearly 

 equal to that of the writer's bi-quartz wedge plate. ^ 



The bi-quartz wedge plate is, however, sniperior to the present 

 device because its action is independent of the direction of the 

 Une of junction between the two halves of the plate. The 

 adjustment of the tilting glass plates is, however, not difficult 

 between crossed nicols and this form of half shade apparatus 

 has the advantage of slight cost and easp of preparation, it being 

 possible to construct the complete apparatus in two or three 

 hours time. The differences in angle of rotation for different 

 wave lengths for a definite position of the apparatus are small 

 and practically negligible for most work, the result being, that 

 even for large angles of tilting of the apparatus, the field remains 

 uncolored and of a light gray hue. In this respect it differs 

 from the bi-quartz wedge plate and for certain purposes may be 

 superior to it. 



PHYSICS.— A transmission and reflection photometer for small 

 areas. P. G. Nutting and L. A. Jones. (Communication 

 No. 15 from the Research Laboratory of the Eastman 

 Kodak Company.) 



In many optical investigations it is desirable to measure the 

 brightness of small areas — 1 mm. square or less. The instru- 

 ment here described was devised to measure the brightness of 

 optical 'mages, the local densities in photographic negatives, and 

 the reflecting powers of different parts of photographic prints 

 and other pictures. It has proved exceedingly convenient and 

 precise, and so nearly all that may be hoped for in this type of 

 photometer that a description may be of general interest. 



The chief advantages secured in our instrument are the 

 following : 



1. The elimination of errors due to fluctuation in the comparison 

 source by using the same source for comparison light and for the source 

 of transmitted or reflected light. 



1 F. E. Wright, Am. J. Sci. (4), 26: 377-378. 1908. Carnegie Inst, of Wash- 

 ington, Pub. 158: 139. 1911; see also M. Berek, Neues Jahrbuch f. Mineral., B. B., 

 33: 583-661. 1912. 



