TELESCOPES. 07 



eifiployment, hy Roiuer, of great meridian telescopes in 1691, 

 observations of the stars by day have been frequent and fruit- 

 ful in results, having been, in some cases, advantageously 

 applied to the measurement of the double stars. Struve 

 states* that he has determined the smallest distances of ex- 

 tremely faint stars in the Dorpat refractor, with a power of 

 only 320, in so bright a crepuscular light that he could read 

 with ease at midnight. The polar star has a companion of 

 the ninth magnitude, which is situated at only IS" distance : 

 it was seen by day in the Dorpat refracting telescope by 

 Struve and Wrangel,t and was in like manner observed on 

 one occasion by Encke and Argelander. 



Many conjectures have been hazarded regarding the cause 

 of the great power of the telescope at a time when the dif- 

 fused light of the atmosphere, by multiplied reflection, ex- 

 erts an obstructing action. $ This question, considered as an 



* Struve, Mens. Mlcrom., p. xliv. 



t Schumacher's Jahrhuch fur 1839, s. 100. 

 ' t La lumiere atmospherique dijf^ise lie peat s'expliqner par le reflet 

 dt's rayons solaires sur la surface de separation des couches de differen- 

 tes densites dout on suppose ratraosphere composee. En effet, suppo- 

 Bons le soleil place a I'horizon, les surfaces de separation dans la direc- 

 tion du zenith seraif^nt horizontales, par consequent la reflexion serait 

 horizontale aussi, et nous lie venious aucuiie lumiere au zenith. Dana 

 la supposition des couches, aucun rayon lie nous arriverait par voie 

 d'liue premiere reflexion. Ce ne seraient que les reflexions multiples 

 qui pourraient agir. Done pour expliquer la lumiere diffuse, il faut se 

 figurer I'atmosphere composee de molecules (spheriques, par exemple) 

 dont chacune donne une image du soleil k peu pres comme les boules 

 de verres que nous pla9ons dans nos jardins. L'air pur est bleu, par- 

 ceque d'apres Newton, les molecules de l'air out Vipaisseur qui convi- 

 ent a la reflexion des rayons bleus. II est done naturel que les petites 

 images du soleil que de tons cotes reflechissent les molecules sphe- 

 riques de l'air et qui sont la lumiere diff^use aient une teinte bleue: 

 mais ce bleu n'est pas du bleu pur, c'est un blanc dans lequel le bleu 

 predomiue. Lorsque le ciel n'est pas dans toute sa purete et que l'air 

 est mele de vapeurs visibles, la lumiere ditfuse re9oit beaucoup de 

 blanc. Comme la lane est jaune, le bleu de l'air pendant la nuit est un 

 peu verd^tre, c'est-a-dire, melange de bleu et de ,iaune." 



" We can not explain the diffusion of atmospheric light by the reflec 

 tion of solar rays on the surface of separation of the strata of different 

 density, of which we suppose the atmosphere to be composed. In fact, 

 if we suppose the sun to be situated on the horizon, the surfaces of sep- 

 aration in the directicui of the zenith will be horizontal, and consequfnt- 

 Iv the reflection would likewise be liorizontal, and we should not be 

 able to see any light at the zenith. On the supposition that such strata 

 exiit, no ray would reach us by means of direct reflection. Repeated 

 reflections would be necessary to produce any effect. In order, there- 

 fore, to explain the phenomenon of diffused light, we must suppose tha 

 atmosphere to bo composed of molecule.^ (of a spherical form, for in 



