2i8 THE POPULAR SCIENCE MONTHLY 



very complete. In addition to the mimerons 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 spectro- 

 scope 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 down- 

 ward 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 struc- 

 ture which supports the coelostat at the top is completely sheathed in an 

 encasing tower which supports the dome, so that there is complete pro- 

 tection from the wind. When one looks at the tower he sees only the 

 framework of the sheathing. This great tower telescope is a most effi- 

 cient 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. Eitchey in 

 the shop at Pasadena. The remarkable photographs of nebulse 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 com- 

 pletion. The figuring of the enormous block of glass has also been done 

 by Mr. Eitchey. The present state of the building to hold this great 

 reflector is shown in the accompanying picture. The comi^letion 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 nebulae await dis- 

 covery " 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 an- 

 nual 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 Eeport (1913) seventy-two results are 

 summarized. Most of these are of such a technical nature that they are 



