1886.] on Telescopic Objectives and Mirrors. 429 



terminating in a small chamber for eye-pieces and observer. About 

 half-way in the long tunnel there will be a branch pipe connected 

 to the air shaft of the fan, which is used regularly for blowing the 

 blacksmith's fire, and through this when desired a current of air 

 can be sent to " wash it out " and mix up all currents of varying 

 temperature and density. It may be found necessary even to keep 

 this going during observations. 



By this arrangement I hope to be able to have trials whenever 

 required, instead of having to wait hours and days for a favourable 

 moment. 



Figuring of Plane Mirrors. 



There is a general idea that the working of a plane mirror or 

 one of very long radius is a more difficult operation than those of 

 more ordinary radii. This is not exactly the case. There is no 

 greater difficulty in figuring a low curve than a deep one, but the 

 difficulty in the case of absolutely plane mirrors consists simply in the 

 fact that in their figuring there is one additional condition to be 

 fulfilled, viz. that the general radius of curvature must be made 

 accurate within very narrow limits. In figuring a plane mirror to 

 use, for instance, in front of even a small objective, say 4-iuch aperture, 

 an error in radius which would cause a difference of focus of ywo 

 of an inch would seriously injure the performance. This would be 

 about equivalent to saying that the radius of curvature of the mirror 

 was about 8 miles, the versed sine of which with the 6 -inch 

 spherometer would be about -50-^0^ of an inch. Now what I mean 

 to convey is this ; that it would be just as difficult to figure a convex 

 or concave lens of moderate curvature as a flat lens of tho same size 

 if it were necessary to keep the radius accurate to that same limit, 

 i. e. one-tenth of a division of this spherometer. 



Lick Observatory. 



For the final testing of large objectives or mirrors, it is necessary 

 to have them properly mounted, and in such a manner that they can 

 be directed conveniently on any celestial object, and kept so directed 

 by clockwork to enable the observer to devote his whole attention 

 to the testing. I had not intended touching at all on the subject 

 of the mounting of telescopes, but I have been asked to call your 

 attention to this model of a proposed observatory for Mount Hamilton, 

 California, as it embraces some novel features, but I shall do so in 

 only a very few words. 



Most here are probably aware that a monster observatory is in 

 course of erection in California, a large sum of money having been 

 left for the purpose by a Mr. Lick. The observatory is already 

 partly complete, and contains some excellent instruments of moderate 

 size, the work already done with which warrants us to hope that the 

 great 3 6 -inch refractive about to be erected will be placed under 



