Eclipses 



all the facts observed, he attributed a thickness 

 of sixty-four leagues to the lunar atmosphere. 

 But the absence of any gaseous layer around 

 the moon is now, and has long been, beyond 

 dispute; it is sufficient to follow the course of 

 the stars when they pass near our satellite to 

 realise, with all the precision of astronomical 

 measurement, that the trajectory of their rays 

 is not in the slightest degree deviated, as it 

 would be by an atmosphere, were it ever so 

 shallow or attenuated; and the distinctness of 

 the umbra in eclipses of the sun is sufficient to 

 prove beyond doubt the absence of any gaseous 

 layer around our satellite. 



At the time when Louville was bringing 

 forward the " air of the moon/' other astro- 

 nomers thought the corona might be attributed 

 to a diffraction-phenomenon. Luminous rays 

 are not propagated rigorously in straight lines, 

 but when they meet an obstacle the separation 

 of light and shadow is less distinct than would 

 be imagined from geometrical optics; this 

 property of light explains the following experi- 

 ment carried out in 1715 by Delille. A pencil 



of sunlight was admitted into a camera obscura 



205 



