286 PROCEEDINGS OP THE AMERICAN ACADEMY. 



no mechanical vibrations due to heavy machinery, which would not have 

 been the case if the current had been generated in the building. 



Reduction of Astigmatism, Method of Measuring Plates, etc. 



The investigation was confined to the second spectrum, the dispersion 

 being one Angstrom unit to 0.75 mm. The length of the rulings of the 

 grating surface was diminished by reducing the vertical aperture to about 

 2 cm. (using black paper for the purpose) ; this process, together with 

 stopping down the length of the slit to 5 or 6 mm., so much reduced the 

 astigmatism that the tips of the spectral lines could be seen in the field 

 of the micrometer used in measurement. We thus avoid a shift due to 

 over-exposure, for, by setting the cross-hair upon the tip of the spark 

 image, where it is just visible to the eye, we are certainly making use 

 of minimum exposure conditions. 



The filar micrometer (manufactured by Bausch & Lomb) was re- 

 inforced by adjustable trusses to produce greater rigidity. 



A test of the accuracy of the instrument showed that the errors of 

 measurement were greater than those of the screw. The magnification 

 employed was about seven diameters, and a single cross-hair was found 

 to give the best results. The plates were placed violet left, and four 

 settings were made on the external or spark spectrum, then two upon the 

 interna] or arc spectrum ; then the same set of measurements was repeated 

 with the violet to the right. The plates used were the regular seed 

 " Gilt Edge " 27, 5x7, cut into strips I{x7 inches. 



The developer generally used was that described by Wallace * as giving 

 with the seed 27 plate a definite and regular silver grain deposit. 



Several plates were developed with Jewell's solution,! to which was 



* Astrophysical Journal, 20, 118 (1904). The formula was kindly sent the au- 

 thor by Mr. Wallace, and is as follows : 



A. Hydrochinon ... 1 gm. B. KBr ... 1 gm. 



Metol 1 gm. NaOH . . 12 gms. 



Adurol 2 gms. H 2 . . . 175 gms. 



Na 2 S0 3 (crystals) . 24 gms. 



HoO 175 gms. 



Take equal parts of A and B. 



t A. Hydrochinon. . 30 gms. B. Potas. ferro- C. K 2 C0 3 30 gms. 



Na 2 S0 3 (crystals) 150 gms. cyanide. 80 gms. H 2 . 180 gms. 



H 2 750 gms. H 2 . . . 180 gms. 



Alcohol. . . . lOc.c. 

 Take one part of B, one of C, and six of A. Normal amount of bromide, 15 

 drops (10 per cent solution) to 100 c.c. of developer. 



