340 



T H E G U I J ) E TO NATURE 



the result of the previous days work 

 and to correct the adjustments of the 

 tools. These tests were made with a 

 6o-inch optically plane mirror, and the 

 grinding of this mirror was in itself a 

 huge task and took nearly a year. Over 

 600 of these tests were made during 



This was constructed in the form of a 

 revolving plate as large as the glass 

 itself and weighing 1500 pounds. It 

 was so arranged that it would exert 

 an even pressure on the entire surface 

 of the glass and so corrected the ten- 

 dency to astigmatism. 



THE TOOL USED FOR APPLYING THE EMERY FLOUR IN POLISHING THE GREAT MIRROR. 



THE WORKING MODEL OF THE MOUNTING OF THE 100-INCH REFLECTOR. 



the three years of polishing. One very 

 serious difficulty was encountered 

 early in the polishing. It was found 

 that the curves of concavity were not 

 the same at right angles to each other 

 or, in other words, there was astigma- 

 tism in the glass. The .0016 of an inch 

 by which they differed threatened to 

 render the glass useless, but after sev- 

 eral months of work the problem was 

 solved by the use of a new kind of tool. 



Early in the spring of 1917 the most 

 rigorous tests failed to show any im- 

 perfection in the concavity of the sur- 

 face and the glass was made into a 

 mirror by chemical precipitation of 

 silver from a solution poured over the 

 surface. 



The task of transporting the four 

 and one-half tons of glass up a moun- 

 tain road, where it would be subject 

 to jar and sudden changes of tempera- 



