74 



proceedings: philosophical society 





Humphreys referred to the assumption of three regions of dustiness 

 which offers the easier explanation of sky polarization measurements, 

 viz: (1) lower atmosphere hardly reaching the altitude of Mt. Wilson 

 and a region of large dust particles ; (2) region of smaller dust particles 

 extending from (1) to an altitude of about two miles, about the height 

 of the ordinary convection clouds, and (3) thence to highest limit of 

 convectional atmosphere and under surface of isothermal region and 

 beyond, ordinarily very free of dust; region (1) would be most effective 

 in scattering. Mr. Fowle noted that the losses at altitudes above 1000 

 meters were practically nothing, probably \ per cent. Messrs. C. A. 

 Briggs and Priest referred to certain anomalous halo phenomena 

 which Mr. Humphreys explained as well-known cases discussed in 

 works on meteorological optics. 



Mr. I. G. Priest then spoke on A simple spectral colorimeter of the 

 monochromatic type. The possibility of so-called "monochromatic 

 color analysis" is due to the experimentally observed fact that any color 

 sensation (except purples) can be "matched" by the sensation caused 

 by a suitable mixture of some one monochromatic light with white 

 light. From a philosophical point of view, this is the most natural and 



the simplest method of denning a color, but 

 I A C J the experimental difficulties and uncertain- 



ties of putting it into practice have been 

 considerable. The chief trouble has been 

 due to the necessity of making a photo- 

 metric comparison between colored light and 

 white light in each color determination. 

 The purpose of the present device is to 

 eliminate this difficulty. The novel and 

 L B D K essential feature of the instrument is the 



system of slits shown in the diagram. 

 A B C D is a bilateral slit the width of which can be adjusted and 

 measured by a micrometer screw. E F G H is a slit cut in the jaws of 

 slit A B C D and perpendicular to it. (Actually, the jaws of slit A B 

 C D consist of four wings, adjustable on two plates so that the width 

 of the slit E F G H can be adjusted by sliding.) I J K L is a bilateral 

 slit similar to A B C D with its jaws sliding on the jaws of the latter and 

 placed so that their center-lines are coincident. This slit system is 

 mounted in the focal plane of the collimator of a spectroscope so that 

 the slit E F G H has the ordinary position of a spectroscope slit ; and 

 provision is made for moving the whole system in its own plane, in 

 the direction A B in such a way that the displacement can be 

 measured. The whole slit system is illuminated by a uniform white 

 diffusing surface. A slit parallel to the direction E F is placed in the 

 focal plane of the observing telescope of the spectroscope; and obser- 

 vations are made with the eye at this slit. "Dominant hue" is varied 

 by displacement of the slit system in the direction A B and is deter- 

 mined by the position of the slit E F G H. "Purity" is varied by 

 varying the relative widths of the slits A B C D and I J K L and is 



MN 

 



-F 



