Intelligence and Miscellaneous Articles. 387 



ON THE DETERMINATION OF THE COEFFICIENTS OF INTERNAL 



FRICTION. BY WALTER KONIG. 



The author determined the coefficients of a number of liquids by 



the method of vibrating disks, and also by that of flow from capillary 



tubes, and obtained numbers which agree with those obtained by 



other inquirers. 



He also examined whether electrification and exposure- in a 

 magnetic field had any influence on this coefficient, and obtained a 

 negative result. — Wiedemann's Annalen, No. 8, 1885. 



NOTE ON THE TRANSMISSION OF LIGHT BY WIRE-GAUZE 

 SCREENS. BY S. P. LANGLEY. 

 In the beginning of the present year a friend sent me a series of 

 wire-gauze screens, which he used to diminish the apparent bright- 

 ness of stars in making meridian observations, with a request that 

 I would determine photometrically the amount of light transmitted 

 by them. As such screens are occasionally employed in astronomical 

 work, particularly in the use of the heliometer, I have thought the fol- 

 lowing account of our experience of sufficient interest to make public. 

 I used for the measurements a photometer-box originally con- 

 structed for another purpose, and an opaque wheel or disk having 

 radial slits of variable width, which, placed in the path of a ray of 

 light and rotated with sufficient velocity, can be made to reduce the 

 light to any desired fraction of its original intensity. (This I have 

 employed for some years for photometric measurements when it is 

 desirable to avoid the use of polarizing-apparatus.) In the centre 

 of the photometer-box was a sliding Bun sen disk, which could be 

 viewed from above by a suitable arrangement of mirrors. The 

 open ends of the box were directed to two opposite windows, and the 

 disk placed in such a position that its sides were equally illuminated. 

 The wire screen was then placed over one end of the box, the wheel- 

 photometer in front of the other end, and the apertures of the 

 latter altered until the equality of illumination of the Bunsen disk 

 was restored. The screen then cut off the same amount of light as 

 the wheel. Frorn several series of measurements made in this way, 

 it was found that 



1 screen transmitted -395 -f -004 of the incident light. 



2 screens superposed transmitted '144 + -004 „ ,. 



3 „ „ „ -052 ±-003 „ „ 

 These numbers, as was to be expected, are nearly in geometrical 



progression. The screens were returned to the sender and the 

 results communicated to him ; but he wrote that, upon trial, he 

 found the reduction of light very much greater than the above 

 values, three superposed screens reducing the light of a star by 7*1 

 magnitudes, which corresponds to a transmission of only -0014. 



I was at that time absent, and my assistant, Mr. J. E. Keeler, 

 undertook the investigation of the cause of the discrepancy, which 

 he attributed to loss of light by diffraction under the circumstances 

 in which the screens were used by their owner, i. e. in front of the 

 object-glass of a telescope directed upon a star. With diffuse 



