'nM.i'.S(^(U'i'.<. r»7 



ei'iiployineul, by Koiiut. oj'gri'tit meridian ti-k'.-c.ojjes in IG'Jl, 

 observations cl" llio stars by day have been IVequeiit and iriiit- 

 fnl 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 Argclander. 



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 Schnmachei'^s Jahrbtick fur 1839, s. 100. 



X La lumiere af.mosphdrique diffuse ne pent s'expliquer par le reflet 

 des rayons solaires siir la surface de separation des couches de diifereu- 

 tes deusites dout on suppose I'atmosphere coniposee. En effet, suppo- 

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

 tion du zenith seraient horizontales, par consequent la reflexion serait 

 horizontale aussi, et nous ne venions 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 pouriaient asir. Done pour expliquer In lumiere diffuse, il faut so 

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

 dent chacune donne une image du soleil a pen pres conime les boules 

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

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

 ent a la reflexion des rayons bleus. II e.<t done nature] que les petites 

 images du soleil que de tons cotes rcflechissent les molecules sjihe- 

 riques de l'air et qui sent la linniere difluse aient une teinte bleuo : 

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

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

 est mele de vapeurs visibles, la lumiere diffuse recoit lieaucoup de 

 blanc. Coinme la bane est jaune, le bleu de l'air pendant la nuit est un 

 pen verdatre, c'est-a-dire, melange de bleu et de jaune." 



" We can not explain the diffusion of nfmospheric light by the reflec- 

 tion of solar rays on the surface of separation of the strata of difle rent 

 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 l)e horizontal, and consequent- 

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

 able to see any light at the zenith. On the sup[)osition that >uch strata 

 exist, n) 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 o'i diffused light, we must suppose the 

 atmosphere to be composed f)f molecules (of a spherical form, for in 



