TELESCOPES. G7 



employment, by Romer, of great meridian telescopes in 1G91, 

 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 18" 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. Microm., p. xliv. 



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



\ La lumiere atmosph6rique diffuse ne peut s'expliquer par le reflet 

 des rayons solaires sur la surface de separation des couches de differen- 

 tes densites dont on suppose l'atmosphere compos6e. En effet, suppo- 

 sons le soleil place a l'horizon, les surfaces de separation dans la direc- 

 tion du zenith seraient horizontales, par consequent la reflexion serait 

 horizontale aussi, et nous ne verrions aucune lumiere au zenith. Dans 

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

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

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

 figurer l'atmosphere coraposee de molecules (spheriques, par exemple) 

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

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

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

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

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

 riques de l'air et qui sont la lumiere diffuse aient une teinte bleue : 

 mais ce bleu n'est pas du bleu pur, e'est uu blanc dans lequel le bleu 

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

 est m61e de vapeurs visibles, la lumiere diffuse reijoit beaucoup de 

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

 peu verdatre, e'est-a-dire, melange de bleu et de jaune." 



" 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 direction of the zenith will be horizontal, and consequent- 

 ly the reflection would likewise be horizontal, and we should not be 

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

 exist, 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 be composed of molecules (of a spherical form, for in 



