abstracts: physics 325 



interference fringes instead of their displacement as in the Fizeau 

 method. The outHne of the method on this basis is briefly : 



1. One of a pair of nearly, but not quite parallel interferometer 

 mirrors is supported at one point by the sample and at two other 

 points by a standard substance of known expansivity, the perpendicular 

 distance from the bearing point of the sample to the line connecting 

 the bearing points of the standards being known. 



2. The standard and the sample are so adjusted that the fringes 

 are parallel to the line connecting the bearing points of the standard. 



3. Any difference in expansivity of the sample and the standard 

 causes the interference fringes to change their widths with change in 

 temperature. The number of fringes between two fixed reference lines 

 on one of the mirrors is determined at each of the two temperatures in 

 question. 



4. From the above data the difference in the expansion of the standard 

 and the sample can be computed. 



The new method gives results of the same accuracy as the Fizeau 

 method, while it has marked advantages over the latter. I. G. P. 



PHYSICS. — Co«/ra5/ sensibility of the eye. Enoch Karrer and E. 

 P. T. TyndaIvL. Bur. Standards Sci. Paper 366. Pp. 14, figs. 

 10. 1920. 



A knowledge of the contrast sensibility of the eye is very essential 

 to the proper understanding of the theory and use of searchlamps and 

 searchlight illumination. 



The searchlamp is used at night when the eye is generally adapted 

 to low levels of illumination. If the observer is far removed from the 

 searchlamp the illumination may be simply that from the moon and 

 sky. If he is near the lamp, however, he must look through the dif- 

 fused light along the beam. 



In order to be visible, the target must be illuminated to a degree 

 that will make sufficient contrast in brightness or color between it 

 and this surrounding field. Data are given showing the relationship 

 that exists between the brightness and the size of the target and the 

 brightness of the surrounding field. 



In these experiments a large surface painted white was illuminated 

 with an incandescent lamp. The target consisted of a rectangular 

 spot of this surface more brightly illuminated by means of a projection 

 lantern, equipped with a simple bilateral slit. The image of the slit 



