ASTRONOMY — HALE. 6 I 



soon after sunrise and before sunset to secure the daily record, so that 

 the plates may be suitable for investigations of the more minute solar 

 phenomena. 



Images of the stars and of the moon given by the Snow telescope 

 are frequently very fine, but after the mirrors have become distorted 

 through heating by the sun they sometimes do not return to their 

 normal figure for many hours. The images observed at such times 

 are greatly distorted and usually multiple in character. 



The special form of house designed for the Snow telescope has 

 proved very satisfactory. The louvres and ventilated roof undoubt- 

 edly accomplish their purpose, as the air within the house remains 

 cool even at midday. 



QUARTZ MIRRORS. 



The above remarks emphasize the importance of carrying to a suc- 

 cessful issue the experiments on the use of fused quartz for mirrors 

 undertaken last year and reported upon in Year Book No. 3 (p. 127). 

 Professor Gilmore, of Throop Polytechnic Institute, who has carried 

 on the experiments this year in conjunction with Professor Ritchey, 

 succeeded in making quartz disks decidedly better than any obtained 

 in the earlier work. The method of heating the quartz crystals to 

 about 500 C. before dropping them into the white hot electric fur- 

 nace proved more successful than other, methods, in that the disks 

 thus produced contain fewer bubbles. Nevertheless the bubbles still 

 remaining are too numerous to permit a satisfactory optical surface 

 to be given the quartz disks, and experiments in remelting the sur- 

 face in the electric arc have not succeeded, as was hoped, in remov- 

 ing the bubbles from a superficial layer of sufficient thickness for 

 figuring. The molten quartz is far too viscous to permit the bubbles 

 to be removed by stirring, but it is hoped that melting the crystals 

 under pressure may accomplish the purpose. 



Meanwhile there is reason to hope that materials other than ordi- 

 nary glass or fused quartz may prove suitable for mirrors. Schott, 

 of Jena, has produced an opal glass having a coefficient of expansion 

 about one-third that of ordinary glass. Other glass-makers will be 

 induced, if possible, to make special experiments in the hope of pro- 

 ducing glass of low expansion coefficient. 



There is reason to think that speculum metal, on account of its 

 high conductivity of heat, may not undergo in sunlight such surface 

 distortion as glass exhibits. We are preparing similar mirrors of 

 glass and speculum metal for a careful comparative test to decide this 

 question. 



