REPORT OF THE SECRETARY. 67 



A determination has been made of the change in deviation of the A line with 

 change of temperature of the salt prism. The value obtained was — 12.0" j^ 0.4" for 

 the change in deviation, corresponding to 1° rise in the temperature of the prism. 



Interesting results were obtained, experimentally, showing that for slit widths less 

 than 0.15 millimeter the increase in the loss of energy by diffraction is very rapid. 

 An investigation was begun, and bids fair to end successfully, with the design of 

 constructing a galvanometer far more sensitive than the one in present use, which 

 may enable a considerable gain to be made in the number of lines to be discovered 

 by the holographic method of spectrum analysis. 



ACQUISITIONS AND ALTEKA'l'IONS OK APPARATUS. 



1. A concave mirror 26 centimeters in diameter and of 230 centimeters principal 

 focal length was obtained for use in the system of collimation by two mirrors, as 

 proposed in last year's report, and actually installed with advantage in August, 1896. 

 The metal work required in the mirror mountings was done at the Observatory shop. 



2. Three very accurate optical flats, 12 centimeters in diameter, were procured for 

 testing the faces of the salt prism during its occasional polishings, and for use as 

 plane mirrors when required. 



3. A train of totally reflecting prisms, so adjusted as to invert an image, was 

 obtained for use with the 6-inch telescope as an erecting eyepiece. 



4. A number of large blocks of sait were received through the courtesy of the 

 Russian Government, and from one of the blocks, which promised good optical qual- 

 ity, there was cut a large prism. On polishing, however, the quality of the interior 

 was found to be so far below the exterior appearance that the prism has not been 

 made use of. The optical work on the pieces of apparatus above mentioned was 

 done by Mr. J. A. Brashear. 



5. A Zeiss anastigmatic wide-angle photographic lens was procured for use in 

 photographing interior rooms. 



6. A Nachet compound microscope with mechanical stage and various accessories 

 was obtained and is frequently used for measurements of bolometer threads and 

 other small objects. 



7. There have been made at the Observatory shop during the year, in addition to 

 such repairs as were required, the following pieces of apparatus : 



a. Bearing for circle shaft to insure greater accuracy of motion. 



i. A mounting for oblong plane mirror and prism to form an auxiliary fixed-arm 

 spectroscope. This piece of apparatus was used in the determination of the change 

 of deviation of rock salt with change of temperature. 



c. A special rheostat for balancing the bolometer, provided with a slide wire oper- 

 ated mechanically from Avithout, and all inclosed by a water jacket. 



8. It was shown that the circle of the spectrometer moved less accurately than was 

 desired, and that the error arose in the action of the worm-and-wheel mechanism, 

 which connects it with the clock. Messrs. Warner and Swasey undertook the task of 

 replacing the worm and wheel-segment in use, by others of the desired degree of 

 accuracy. In this they were successful, and the degree of accuracy attained may be 

 inferred when it is said that it is required that the position of the circle shall not be 

 in error by more that 0.5 second of arc, which corresponds to a linear distance of 

 0.00003 inch on the circumference where the worm and wheel-segment is applied. 



9. A comparator for the measurement of holographs in ordinates and abscisssB was 

 procured of Warner & Swasey. The specifications were prepared at the Observa- 

 tory with a provisional design which was elaborated and modified by the makers. 



10. The large Zeiss photographic lens has been fitted with a Prosch rapid shutter. 



11. A new motor and a set of small taps and dies have been added to the shop 

 equipment. 



12. On the recommendation of Dr. Kayser, of the University of Bonn, who inspected 

 the Observatory during a visit to this country, sulphuric acid has been used to dry the 



