proceedings: philosophical society 75 



computed from these slit widths, the ocular slit width, the width of 

 EFGH, the "visibility" function, and the dispersion. Comparison 

 with a sample is made by means of a Lummer-Brodhun cube before 

 the objective of the observing telescope. Relative "brightness" is de- 

 termined by relative slit widths. Scale values of slit width A C can 

 be changed by means of neutral absorbing films covering A C N M and 

 B P D and not covering EFGH. This instrument was planned 

 in April, 1915. The new slit system has been constructed, mounted, 

 and tried qualitatively with improvised accessory apparatus. A com- 

 plete instrument adapted to precise work is now to be designed and 

 constructed, after which quantitative tests of its functioning will be 

 undertaken. 



INFORMAL COMMUNICATIONS 



Mr. I. G. Priest exhibited a newly-designed slide-rule for the rapid 

 computation of the ratio tan #/tan X where the angles have values be- 

 tween 6° and 84°; this slide-rule is particularly useful for laboratory 

 reductions of observations with Marten's photometer and the Koenig- 

 Marten spectrophotometer. Mr. Priest also exhibited a graphical 

 device designed to evaluate equations of the form y = x n for values of 

 x and y between 0.01 and unity and for values of n between about 

 0.1 and 10; this device is especially useful and permits rapid compu- 

 tation of transmission for a given thickness when the transmission for 

 some experimental thickness of material is known. 



Mr. J. A. Fleming exhibited a combined magnetometer and earth 

 inductor with a portable galvanometer of the Kelvin type for field 

 use designed and constructed by the Department of Terrestrial Mag- 

 netism of the Carnegie Institution of Washington and spoke of some 

 severe field trials showing the availability of this instrument for such 

 use. The high degree of accuracy attainable was indicated by a resume 

 of recent observatory intercomparisons of magnetic standards using an 

 instrument of this type at Eskdalemuir in Scotland, Kew, Greenwich, 

 and Stonyhurst in England, Cheltenham, Maryland, and Honolulu, 

 Hawaii; these results indicate a readily attainable absolute accuracy 

 of 0'.2 to 0'.3 in declination and inclination and about one five-thou- 

 sandth part or less in horizontal intensity. The work reported on is 

 the first practical successful application to field use of the earth inductor. 

 Mr. Bauer, discussing this communication, referred to the great improve- 

 ment effected in magnetic instruments since his comparisons of obser- 

 vatory standards in 1899, and to the work of Schering at Darmstadt 

 about 1899 with an earth inductor and galvanometer of moving-coil 

 type which was used with great inconvenience at field stations but could 

 not be considered practically applicable for field work. 



Mr. F. E. Wright stated that in his work with microscopes he 

 had found that the better position of the plane of vibration of the 

 polarizer is parallel to the vertical cross-hair of the eyepiece when ob- 

 servations are made in the early morning or late afternoon and parallel 

 to the horizontal cross-wire at noon. (Journ. Wash. Acad. Sci. 5: 

 641. 1915.) 



