1 68 CARNEGIE INSTITUTION 



to an elevation of 7,000 to 10,000 feet before proceeding to the more 

 trying height of 14,000 or 15,000 feet. In my own case, very much 

 against my will at the time, I was detained at Arequipa (7,750) feet 

 for a whole week, with the advantageous result that I experienced 

 hardly any inconvenience when from there I went on to Vincocaya, 

 at a height of 14,360 feet. I mention this in particular, as such 

 very unfavorable reports are current regarding the railway journey 

 on the Oroya line from Lima and the mortality among the workmen 

 employed in constructing that very remarkable railway. 



As regards the desirable instrumental equipment, there is one 

 point which I desire to emphasize. The mirrors of the reflecting 

 instruments should be made of speculum metal and by no means of 

 silvered glass. Polished silver is incomparably more liable to tarnish 

 than good speculum metal. Besides, even what would be called a 

 good film of silver on glass is in a considerable degree transparent 

 to ultra-violet rays, as was pointed out by Stokes and Cornu many 

 years ago (A?males de V Ecole Normale Supericurc, ser. 11, tome ix, 

 1880, pp. 22-23). Respecting the behavior of such a film with 

 regard to the infra-red rays I have no knowledge, but doubtless 

 your own Professor Langley has had abundant experience on this 

 point. On the other hand, mirrors of speculum metal reflect the 

 low grade heat rays of the moon, and all other rays up to the ex- 

 treme known limits of the ultra-violet, with apparent equal com- 

 pleteness. Moreover, a mirror of speculum metal, when made of 

 the proper alloy and well polished, is, under proper care, one of the 

 most permanent of known optical appliances. I have before me the 

 Cassegrain mirror of a reflecting telescope of 6 inches aperture, made 

 by Short in 1745 ; both it and the other mirrors of that telescope 

 are, to use the words of Dr. Dreyer, ' ' as bright as if they had been 

 polished yesterday." I must add, however, that the telescope, 

 which formerly belonged to King George III, and is now at the 

 Armagh Observatory, seems to have been but rarely used. We 

 have here, however, a Gregorian reflector made by Cary something 

 over 100 years ago, which is frequently used by us for watching the 

 timeball, and though never repolished, is still so bright that one 

 would hesitate to relegate it to the polisher. We have also the mag- 

 nificent 5^2 -inch grating presented by the late Professor Rowland 

 more than 20 years ago, also made of speculum metal, which is prac- 

 tically as good as when it was first received by Lord Crawford, though 

 it has been in very frequent use. During one course of observations 

 this grating was exposed to the fumes from peat}' ground, which we 



