MOUNT WILSON SOLAR OBSERVATORY. 223 



PHYSICAL LABORATORY. 

 Instruments. 



Additions to the laboratory equipment include a 4-inch plane grat- 

 ing of exceptionally fine qualit}", ruled by Dr. Anderson on Rowland's 

 machine, and a number of accessories for the 30-foot spectrograph. A 

 set of three 500-volt generators provides a higher direct-current volt- 

 age than was formerly available. The vertical spectrograph for the 

 15-foot concave grating has been completed in the instrument shop 

 and is now being mounted in the laboratory, where it will usefully 

 supplement the plane-grating instrument in certain classes of work. 



Tests have been made in cooperation with Professor Koch on the 

 availability of the registering micro-photometer for the measurement 

 of line intensities in photographs of the Zeeman effect, furnace, and 

 tube-arc spectra. The results are so satisfactory that the method 

 will be generally employed in the laboratory work. 



In connection with his investigations of the Zeeman effect, Mr. 

 Babcock has made a critical examination of the performance of the 

 30-foot spectrograph, involving a careful test of the 8-inch photo- 

 graphic lens and resolving power tests on the 8-inch Michelson grat- 

 ing. The color curve of the lens was first examined as carefully as 

 possible, and a large plane mirror, known to be flat to one-twentieth 

 of a wave, was then placed below it. An examination of the diffrac- 

 tion pattern for pinhole and slit sources led to the following conclu- 

 sions : On the axis the whole lens is practically perfect ; for work off 

 the axis the performance is improved by stopping down the aperture 

 to 6 inches; for actual use in the spectrograph, an inclination of the 

 lens sufficient to throw the reflection from its surface off the photo- 

 graphic plate involves a serious sacrifice in definition. These reflec- 

 tions do no harm in ordinary arc and furnace work, but are trouble- 

 some when the sun is used ; in case of need they can be practically 

 eliminated with the loss of a small amount of light. Tests of the 

 Michelson grating with the absorption spectrum of iodine gave about 

 90 per cent of theoretical resolving power in the third order spectrum 

 when the area of the grating is cut down to 15 cm. by 5.5 cm. 



Mr. Babcock has also determined the color curves for the visual 

 and photographic objectives of 13 feet focal length, and has prepared 

 a chart which makes a selection of the proper slit-width for anj' given 

 purpose extremely easy and rapid. A test was also made of a large 

 Zeiss prism in the 13-foot spectrograph, in the hope that in the green 

 and yellow the prism would prove much faster than the second order 

 of the Anderson grating, thus aiding in the study of the Zeeman 

 effect for very weak lines. It was found, however, that the intrinsic 

 brightness of the second order grating spectrum is about the same as 

 that of the prism and the latter was therefore rejected for the present 

 purpose. 



