ASTRONOMY — HALS. 83 



The Study of laboratory photographs, referred to in paragraph 4, has 

 been done for the most part by Mr. Adams. He has also given much 

 attention to the study of the sun-spot lines and the measurement of 

 photographs taken in connection with his spectrographic investigation 

 of the solar rotation. 



CONSTRUCTION DIVISION. 



The work of the construction division, under the superintendence of 

 Mr, Ritchey, has been confined, for the most part, to the design and 

 building of instruments and the work of widening the New Trail. A very 

 large amount of work has been accomplished, however, and the comple- 

 tion of the Mount Wilson road will permit the resumption of building 

 construction on Mount Wilson in the spring of 1907. The heavy expense 

 of transportation over the present trail has made it seem advisable to 

 postpone such work until the new road becomes available. 



60-lNCH Reflector. 



The work of polishing and figuring the 60-inch mirror, which was 

 undertaken in the autumn of 1905, has progressed almost to completion. 

 As the greatest precautions were necessary in order to avoid danger from 

 scratches, changes of figure due to temperature variation, etc., much time 

 was devoted at the outset to the perfection of the machinery and ap- 

 pliances of the optical shop. The room in which the large grinding 

 machine stands is provided with double windows, sealed so as to prevent 

 the admission of dust. On account of the considerable fall of tempera- 

 ture at night during the cooler months, an automatic heater was installed 

 for the purpose of maintaining the air in the room at practically constant 

 temperature. The air is filtered before it enters the room, the walls and 

 ceiling are shellacked, canvas is suspended above the grinding machine, 

 the floor is kept wet during the polishing, and various other precautions 

 are taken to reduce the danger of scratches. They have proved most 

 successful, and the perfect polish of the glass surface leaves nothing to be 

 desired. An important development of the year has been the perfection 

 of a method of parabolizing, by Mr. Ritchey, which completely eliminates 

 the necessity of any hand work. Thanks to this method, the paraboliz- 

 ing has already been pushed so far that the completion of the work will 

 require but a short time. 



In order to obtain a thoroughly satisfactory test of the figure, Mr. 

 Ritchey decided to make a plane and a concave mirror, each 36 inches 

 in diameter. By means of the parallel rays obtained with the aid of 

 such a plane mirror, the final tests of the paraboloid will be made. A 

 grinding machine for this work was built during the year, and the two 

 mirrors are already well advanced. The convex mirrors for the 60-inch 



