SOLAR OBSERVATORY. ^49 



erecting and riveting this building, as shown in plate 8. In view of unfore- 

 seen delavs, occasioned in part by labor troubles and strikes, and in part by 

 the difficulties of transportation over the mountain road, the dome can not be 

 entirely completed before the rainy season, as had been hoped. This will 

 prevent the erection of the 6o-inch reflector on Mount Wilson before the 

 spring of 1908. 



Toii'cr Telescope.— In Contributions from the Solar Observatory No. 14 

 a plan is described for a vertical coelostat telescope which T devised last sum- 

 mer as a result of the experience gained from the use of the Snow telescope. 

 This instrument, which has recently been completed, is shown in plate 9. 

 It consists of a steel tower 65 feet in height, which carries a coelostat and 

 second mirror, from which the sunlight is reflected vertically downward 

 through a 12-inch objective of 60 feet focal length, mounted near the summit 

 of the tower. The solar image is thus formed a short distance above the 

 ground level, where it enters the slit of a Littrow grating spectrograph of 

 30 feet focal length, wdiich stands in a cylindrical concrete well 8.5 feet in 

 diameter, constructed in the earth immediately under the tower. The instru- 

 ment includes several novel features, among which may be mentioned 

 extremely thick mirrors, to reduce distortion by sunlight ; arrangements for 

 reflecting sunlight on the silvered backs of the mirrors, to compensate 

 residual bending; great elevation of the coelostat, to reduce the effect of 

 disturbances caused by heated air rising from the ground ; the use of a 

 vertical light beam instead of a horizontal one; the provision of a subter- 

 ranean laboratory, nearly constant in temperature, for the spectrograph, etc. 



On account of the pressure of w^ork in our instrument shop, the coelostat 

 and second-mirror support were obtained from Brashear and the Littrow 

 spectrograph from Gaertner. All other work on the instrument was done 

 by our own men, however, including the driving mechanism for the spectro- 

 heliograph, rails for the coelostat, etc. As remarked above, the coelostat and 

 second mirrors were also made in our optical shop. 



The preliminary tests of the tower telescope show that it accomplishes the 

 purposes for which it was designed, i. e., the mirrors change their figure so 

 slowly that long exposures can be given with the 30-foot spectroheliograph, 

 and the definition is at all times better than with the Snow telescope. Excel- 

 lent photographs of the solar spectrum (at center and limb) have been 

 obtained in the third and fourth orders of the 30-foot spectrograph. 



Other Construction. — As usual, much miscellaneous work was done in the 

 instrument shop during the year. This included the building of an addition 

 to the erecting-house, to contain the electric truck ; the reconstruction of the 

 motors and other parts of the truck ; the erection of an addition to the 

 carpenter shop, fitted with a wood planer, for the construction of the 100-inch 

 grinding machine ; considerable work in connection with the Hooker build- 

 ing; minor apparatus, repairs of instruments, etc. 



