Astronomy 605 



sulphuretted hydrogen) is much greater than one might 

 think. Grinding and poHshing glass are treated in several 

 sections also. Rolled glass, such as was used for speculums, 

 has axes of rigidity, which must be considered in mounting 

 the mirrors. The effects of heat in deforming the image 

 formed by a reflecting surface are next studied. The grind- 

 ing-tools and the emery used with them are described, and, 

 again, many points of practical value are brought out. The 

 method of testing the optical surface is fully explained in de- 

 tail. A section of the paper is devoted to the description of 

 grinding-machines of various kinds. This is followed by sec- 

 tions relating to eyepieces, plane mirrors, telescope-mounting 

 (altazimuth), moving plate-holders for photographs, driving- 

 clocks, cameras, the construction of an observatory, observing 

 chair, the photographic laboratory, enlarging apparatus, mic- 

 roscopic photography, and the like. 



In each and all of these sections, it is clear to the reader 

 that he is obtaining the results of first-hand experience ac- 

 quired not by one but by many trials. 



After completing the i5>^-inch mirror, Doctor Draper con- 

 structed a 28-inch Cassigranian reflector, which he mounted 

 equatorially (on a stand constructed by himself) alongside 

 of an 1 1 -inch photographic refractor by Alvan Clark & 

 Sons. The reflectors were at first employed in producing a 

 large number of excellent photographs of the moon, and later 

 in researches in stellar spectroscopy. 



Since Doctor Draper's too early death these and other in- 

 struments have been presented by Mrs. Draper to the Harvard 

 College Observatory. A large annual grant of money is 

 made by Mrs. Draper for the continuation and extension of 

 the researches begun by her husband, and under the skilful 

 direction of Professor Pickering, the publications of the Draper 

 Memorial have already been of high value to science. The 



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