182 CARNEGIE INSTITUTION OF WASHINGTON. 



at Lakeside, close to the edge of the path; second, the use of the large inter- 

 ferometer frame and polar axis as a mounting for the instruments to be 

 employed at the central station. Reference was made in the last annual 

 report to the 50-foot interferometer telescope designed by Hale and Pease 

 for an extension of the work upon the measurement of star diameters. The 

 central section of the 50-foot beam, which is mounted upon a polar axis and 

 moved by a large driving-clock, makes an excellent support for the long 

 camera tubes, spectrographs, interferometers, and other apparatus which will 

 be used at the eclipse. All of these instruments will thus be pointed directly 

 at the sun and the use of coelostats or other reflecting devices will be avoided. 

 Through the kindness of the Chief of Coast Artillery, permission has been 

 received to erect this mounting within the military reservation at Fort 

 Rosecrans, Point Loma, where excellent transportation facilities are available 

 and where local observations give good reason to expect favorable weather 

 conditions. 



Before we turn to a consideration of the scientific work of the year, atten- 

 tion should be called to the important progress which has been made in con- 

 nection with the ruling-engine and the promise which this yields of securing 

 large diffraction gratings, not only of excellent quality but also of exceptional 

 brightness in any desired order. It would be difficult to mention any single 

 advance of an instrumental character which could contribute in a more 

 important way to the success of many of our instruments. The powerful 

 spectrographs in use on the sun under present conditions necessitate exposure 

 times which are longer than they should be for the best results on the finer 

 details of sun-spots or the bright lines of the chromosphere. This is even 

 more true in the case of the grating spectroheliograph, and numerous attempts 

 to secure large prisms have shown the great difficulties encountered by the 

 manufacturers of optical glass in securing material suitable for this purpose. 

 The large ruling-engine which has been under construction in our instrument 

 shop for several years is now beginning to yield results which appear certain 

 to improve this situation very materially. After overcoming the numerous 

 obstacles inherent in the construction of an engine of this size in which the 

 mechanical requirements are so exacting, our instrument maker, Mr. Jacomini, 

 who, with the advice and assistance of Dr. Anderson, has had the entire 

 charge of the building of the engine and its adjustment, has succeeded in 

 ruling several gratings of good quality and remarkable brightness in certain 

 orders. This result has been secured through the remarkable success which 

 Mr. Jacomini has attained in the grinding of diamond points to the angle 

 required to concentrate light in any specified order. The angles may be 

 computed readily, but it is only very rarely that natural diamond points can 

 be found with angles approaching these values. By the use of a simple 

 grinding-machinc designed for this purpose, it has been found possible to 

 shape diamonds to the required form quickly and accurately. Such diamond 

 points are found to be much more free from irregularities than are the natural 

 points, and as a result are considerably more durable on the average. Among 

 the plane gratings which have already been made are several with ruled sur- 

 faces 13 by 8 cm., in which an especial attempt has been made to concentrate 

 the brightness in either the first or the second order. One of these which has 

 been tested in the 75-foot spectrograph of the 150-foot tower telescope permits 

 a reduction in exposure time to one-twelfth that necessary with the excellent 



