14 The Ohio Journal of Science [Vol. XVI, No. 1, 



machine to fit one of the surfaces of the lens, is held in the 

 head A. If the lens be cemented to this with a thin coat of 

 pitch, it is obvious that the surface of the lens next to the 

 holder will have its center of curvature coincide with the axis 

 of rotation of the spindle of the head A, but the center of 

 curvature of the other lens surface will probably fall outside 

 of this axis. A lamp, L, has a tin chimney M'ith a pin hole in it 

 turned towards the lens, this pin hole forming a brilliant point 

 of light, an image of which is formed by each surface and 

 reflected by the total reflecting prism, P, into the telescope, T, 

 where it is seen through the eyepiece. If the centers of cur- 

 vature of both surfaces do not accurately coincide with the axis 

 of rotation of the head. A, the images of the pin hole will 

 describe circles as this axis is rotated. The back surface will 

 of course be centered if the layer of the pitch used as cement is 

 of uniform thickness which will generally be the case if the 

 work has been carefully done ; but in any case the image formed 

 by it should be examined. If the front surface is out of center, 

 as it generally will be, the holder should be warmed and the 

 lens shifted, care being used to keep it tight against the surface 

 of the holder as it is being shifted. As soon as both images 

 remain stationary as the head. A, is rotated, the lens is fed 

 against the wheel, B, and ground true and to size. This 

 worked beautifully and the tests were wonderfully sensitive. 

 As soon as the component lenses of the objective have all been 

 thus centered, they are ready to be assembled in the cell or 

 shutter in which they are to be used; but as this is simply a 

 matter of careful machine work, I need not describe it further. 



I know of no literature on the grinding of small lenses though 

 the following memoirs on the making of large reflecting tel- 

 escopes should be in the hands of any one interested in this 

 work : 



On the Construction of a Five-foot Equatorial Reflecting Telescope. By A. A. 



Common, LL. D., F. R. S. Memoirs of the Roval AstroRomical Society, 

 Vol. L., lSOO-91. 



On the Construction of a Silvered Glass Telescope, Fifteen and a Half Inches in 

 Aperture, and its Use in Celestial Photography. By Henry Draper, M. D., 

 Smilhsonian C'dntrihutions to Knowledj^e, Vol. '-i-i. 



On the Modern Reflecting Telescope and the Making and Testing of Optical Mirrors. 

 By George W. Ritchey. Smithsonian Contributions to Knowledge, Vol. 34. 



