454 Sir George H. Darwin [April 26, 



rosin about a quarter of an inch thick and an inch square are made, 

 and these are glued in rows on the convex face of the wooden tool, 

 with a narrow space intervening between each rosin square and its 

 neighbours. The tool is then w-armed slightly so as to soften the 

 rosin a little, and it is then pressed lightly on to the glass disk. By 

 means of this " warm-pressing " a nearly perfect fit is attained. 



Each of the rosin squares is then painted with hot melted wax. 

 This is done because wax is harder than rosin and affords a better 

 working face. Finally, when the tool is quite cold, the surface of 

 the glass is painted all over with very finely powdered rouge and 

 water, and the tool is placed gently on the glass with some additional 

 weight resting on it. It is left thus for several hours, but is moved 

 slightly every ten minutes to ensure an even distribution of the 

 rouge and water. By means of this " cold-pressing " a perfect fit is 

 secured of the wax-coated rosin squares with the glass face. Cold- 

 pressing has to be repeated every day before the work begins. 



The polishing is now carried on in much the same way as the 

 grinding, but by hand instead of by machine power. The turn-table 

 can be made to tilt so as to bring the glass to stand vertically, 

 instead of horizontally, and the disk is frequently tilted up so as to 

 submit the surface to optical tests. These latter tests are far more 

 searching than those with a spherometer, and enable the observer to 

 detect an error in the radius of curvature of portion of the reflector 

 of a hundredth of an inch. To correct such an error it will be 

 necessary to remove a layer of glass of ^Q-oViTTrtlis of an inch ! 



The most refined optical test is by the observation of the image 

 of a brilliant light issuing from a pin-hole close to the intended 

 centre of the spherical surface. The observer examines the image of 

 the pin-hole with a microscopic eye-piece placed as close as possible 

 to the pin-hole. He then causes a straight-edge close in front of 

 the eye-piece to move slowly across the reflected beam of light, either 

 from left to right or from right to left, so as to eclipse the light. 

 Previously to the eclipse the whole of the glass seems to be a 

 uniform blaze of light, and if the curvature is perfect the light 

 which enters the observer's eye comes from all parts of the disk, and 

 the surface is seen to darken equably all over. But if the surface is 

 imperfect the light from some part is eclipsed sooner than that f]-oui 

 others, and the disk seems to possess considerable hills and valleys 

 illuminated, as it were, by a setting sun. 



The interpretation of these apparent hills and valleys shows where 

 further local polishing with a small tool is requisite. Sir Howard 

 Grubb says that if he suspects a hollow, he holds his hand near the 

 surface for a minute or two ; if a hill is suspected, he washes the 

 region with an evaporating wash. The warmth in the one case and 

 tlie cooling in the other tend to rectify, and indeed over-rectify, the 

 errors. 



When success is finally attained, after all we have only a spherical 



