272 CARNEGIE INSTITUTION OF WASHINGTON. 



engine room. Changes of 0?01 C. over a range of several degrees can 

 be observed. 



A number of special investigations of equipment have been made by- 

 Mr. Babcock, of which the following may be mentioned: 



The absolute equivalent focal lengths of the 30-foot photographic 

 and 13-foot visual lenses have been determined with our best Anderson 

 grating, whose constant is known within 1 part in 3,000. Secondary- 

 standards of wave-length were employed exclusively, using different 

 parts of the spectrum in both second and third orders. Four inde- 

 pendent values of the equivalent focal length corresponding to zero 

 scale reading for the 30-foot lens are 9,249, 9,250, 9,249, 9,250 mm. 

 These are useful for the calculation of the scale of plates which contain 

 no standards of wave-length. 



A comparison of the Kenwood grating with our best Anderson 4-inch 

 grating shows that the former is brighter in the third order, but less 

 perfect in its diffraction pattern. 



The set of three D. C. generators giving 1,650 volts has been con- 

 nected up. Glass and quartz plates have been silvered wdth them by 

 cathodic discharge in a vacuum chamber. Superior results have been 

 obtained by this means, with less labor and uncertainty than is involved 

 in the chemical processes. 



The new interferometer plates of fused quartz recently acquired 

 have been tested by means of the green mercury line. They were 

 found equal to our best glass plates made by Jobin. 



The relative retardations for two of our large compound X/4 plates, 

 one by Werlein for the red and one by Babcock for the blue, have been 

 measured, a wide range in the spectrum being covered in each case. 

 Curves have been prepared showing the relative retardation of the mica 

 as a function of X, from which the percentage of undesirable light 

 transmitted has been calculated. It is found that the two plates suffice 

 for the entire spectrum from X 3500 to X 7000 without introducing more 

 than 2 per cent of undesirable light. 



The Koch registering micro-photometer, now nearing completion, 

 has been installed in position on its pier in the basement of the office 

 building and the mechanical adjustment of its parts is complete. The 

 sensitive string electrometer with its projection microscope is in adjust- 

 ment. It has proved to be unaffected by either mechanical or electrical 

 disturbances in the room since the wiring for the motor has been finally 

 arranged. With a magnification of about 450 diameters, the trace of 

 the filament upon the moving plate is entirely satisfactory for measure- 

 ment. The only important unfinished part of the apparatus is the 

 large plate-holder, which will be constructed after preliminary work has 

 shown the most desirable dimensions. The two screws have runs of 

 21 cm. and 100 cm. respectively, and by means of interchangeable 

 gearing can be so connected that the receiving plate moves 2, 10, 30, 

 or 50 times as fast as the plate which is being measured. 



