very complete. In addition to the numerous smaller pieces of apparatus there may be mentioned in particular the Snow telescope, the two tower telescopes, and the monster reflectors.
The Snow telescope consists of two 24-inch concave mirrors of different focal lengths (when either one is in use the other is easily put out of the way) mounted well above the ground in such a way as to throw the sun's rays horizontally under a louvre covering to the spectroscope or other apparatus, where they are analyzed. Soon after this instrument was in operation Dr. Hale conceived the idea of mounting the coelostat at the top of a tower, and sending the rays vertically downward to the spectroscope so as "to avoid disturbance of definition caused by heated currents of air arising from the ground," He therefore had designed and erected a 65-foot tower for this purpose. This was very successful. Then desiring a greater focal length than could be obtained with this height, be had built a second tower 150 feet high. Under this tower a well was excavated to the depth of nearly 80 feet, thus providing for a possible focal length of about 230 feet. The 150-foot tower is of ingenious construction. It is a tower within a tower. The main structure which supports the coelostat at the top is completely sheathed in an encasing tower which supports the dome, so that there is complete protection from the wind. When one looks at the tower he sees only the framework of the sheathing. This great tower telescope is a most efficient and satisfactory instrument.
There is no larger telescope in operation to-day than the 60-inch reflector, the reflecting surface of which was ground by Mr. Ritchey in the shop at Pasadena. The remarkable photographs of nebulæ that have been made with it speak loudly in praise of its efficiency. This instrument is soon to be supplanted in its proud position of size by the 100-inch reflector, the gift of Mr. J. D. Hooker, which is nearing completion. The figuring of the enormous block of glass has also been done by Mr. Ritchey. The present state of the building to hold this great reflector is shown in the accompanying picture. The completion of this, the largest telescope in the world, will undoubtedly mark an epoch in observational astronomy. Its light-gathering power will be nearly three times as great as that of the 60-inch, and more than seven times that of the Crossley reflector of the Lick Observatory which in its turn fifteen years ago marked an epoch. If "half a million nebulæ await discovery" with the Crossley, think of the possibilities awaiting this giant!
In the ten years of its existence the results of the investigations of the Mount Wilson Solar Observatory have been very numerous and most valuable. I have not space here even to enumerate them. Every annual report of the director contains a summary of the principal results of the year. The number of such results is noticed to increase from year to year. In the last Annual Report (1913) seventy-two results are summarized. Most of these are of such a technical nature that they are
