428 Mr. Hotvard Gruhh [April 2, 



process of local toucli. In this process those parts, and those parts 

 only, which are found to be high are acted upon by a small polisher. 



This action is of course much more severe ; and if only it were 

 possible to know exactly what was required, it ought to be much 

 quicker ; but I have found it a very dangerous j)rocess. I have some- 

 times succeeded in removing a large lump or ring in this way (by 

 large I mean 3 or 4 millionths of an inch), but I have also and 

 much oftener succeeded in spoiling a surface by its use. I look 

 upon the method of local touch as useful in removing gross quantities, 

 but for the final perfecting of the surface I would not think of 

 employing it. 



In small-sized objectives the remedial process is the most trouble- 

 some, but in large-sized objectives the diagnosis becomes much the 

 more difficult, j)artly on account of the rare occurrence of a suffi- 

 ciently steady atmosphere. In working at the Vienna objective 

 it often happened when the figure was nearly perfect that it was 

 dangerous to carry on the polishing process for more than ten minutes 

 between each trial, and we had then sometimes a week to wait before 

 the atmosphere was steady enough to allow of an observation suffi- 

 ciently critical to determine whether that ten minutes' working had 

 done harm or good. It must not be supposed either that the process 

 is one in which improvement follows improvement step by step till 

 all is finished. On the contrary, sometimes everything goes well for 

 two or three weeks, and then from some unknown cause, a hard patch 

 of pitch perhaps, or sudden change of temperature, everything goes 

 wrong. At each step, instead of improvement there is disimprove- 

 ment, and in a few days the work of weeks or months perhaps is all 

 undone. Truly any one who attempts to figure an objective requires 

 to have the gift of patience highly developed. 



In view of the extraordinary difficulty in the diagnostic part of 

 the process with large objectives, it is my intention to make pro- 

 vision which I hope may reduce the trouble in the working of the 

 new 28-inch objective for the Royal Observatory, Greenwich. 



Two of the greatest difficulties we have to contend with are: 

 Istly, the want of homogeneity in the atmosphere, through which we 

 have to look in trials of the objective, due to varying hygrometric and 

 thermornetric states of various portions ; and 2ndly, sudden changes 

 of temperature in the polishing room. The polisher must always be 

 made of a hardness corresponding to the existing temperature. It 

 lakes about a day to form a polisher of large size, and if before the 

 next day the temperature changes 10^ or 15°, as it often does, that 

 polisher is useless, and a new one has to be made, and perhaps before 

 it is completed another change of temperature occurs. To grapple 

 with these two difficulties I propose to have the polishing chamber 

 underground, and leading from it a long tunnel formed of highly 

 glazed sewer-pipes about 350 feet long, at the end of which is placed 

 an artificial star illuminated by electric light ; on the other side of the 

 polishing chamber is a shorter tunnel, forming the tube of the telescope, 



