210 S. P. Langley — Transmission of Light 



Art. XXVII. — 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 brightness 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 following account 

 of our experience of sufficient interest to make public. 



I used for the measurements a photometer box originally 

 constructed for another purpose, and an opaque wheel or disc, 

 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 photo- 

 metric measurements when it is desirable to avoid the use of 

 polarizing apparatus.) In the center of the photometer box 

 was a sliding Bunsen disc 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 disc 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 

 disc was restored. The screen then cut off the same amount 

 of light ns the wheel. From several series of measurements 

 made in this way it was found that 



1 screen transmitted • -395 ±"004 of the incident light. 



2 screens superposed transmitted "144 ±"004 " " 



3 " " " -052±-003 " " 



These numbers, as was to be expected, are nearly in geomet- 

 rical progression. The screens were returned to the sender 

 and the results communicated t.o 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 transmis- 

 sion 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 light such as was used in the measurements 

 with the photometer box, no loss due to this cause was possible. 



