NATURAL PHILOSOPHY. 119 



of which took the audience quite by surprise. It was a view of the 

 moon when about half full, and presented a semicircle of nearly two 

 feet in diameter. The whole of this surface was pictured with great 

 vividness, covered with long ranges of mountains, or dotted with huge 

 volcanic summits into the depths^of which one could look down. From 

 one of these central points, the lava streams could be traced for some 

 eight inches, which, as an inch in length in the picture corresponds to 

 a hundred miles upon the moon's surface, afforded ocular evidence of 

 volcanic agency through a line of eight hundred miles. From another, 

 vast fissures were seen to radiate, which displayed evidence of convul- 

 sions reaching through an area of half that diameter. When it is con- 

 sidered that the largest photographs hitherto obtained of the moon are 

 not more than from four to six inches in diameter, it will be perceived 

 how great a triumph has been achieved by the science and skill which 

 have been requisite for so great an improvement as a picture of twenty- 

 two inches across. 



Foreign Lunar Photographs. Mr. Do la Rue, of London, who has 

 long been successfully experimenting in lunar photographs, has also 

 recently succeeded in obtaining two of these images of our satellite, 39 

 inches (3i feet) in diameter, which he has exhibited to the French 

 Academy. 



THE DIOPTRIC AND ACTINIC QUALITIES OF THE ATMOSPHERE 



AT HIGH ELEVATIONS. 



In a paper on the above subject, read before the British Association, 

 1863, by Professor Piazzi Smyth, he stated, that the chief object of the 

 astronomical experiment on the Peak of Teneriffe in 1856 was to as- 

 certain the degree of improvement of telescopic vision, when both tele- 

 scope and observer were raised some two miles vertically in the air. 

 Distinct accounts have, therefore, already been rendered as to the ma- 

 jority of clouds being found far below the observer at that height, and 

 to the air there being dry, and in so steady a state and homogeneous 

 a condition, that stars, when viewed in a powerful telescope with a high 

 magnifying power, almost always presented clear and well-defined mi- 

 nute discs, surrounded with regularly-formed rings, a state of things 

 which is the very rare exception at our observatories near the sea-level. 

 Quite recently, however, the author had been engaged in magnifying 

 some of the photographs which he took in Teneriffe in 1856, at various 

 elevations, and he finds in them an effect depending on height, which 

 adds a remarkably independent confirmation to his conclusions from 

 direct telescopic observations. The nature of the proof is on this wise : 

 at or near the sea-level a photograph could never be made to show the 

 detail on the side of a distant hill, no matter how marked the detail 

 might really be by rocks and cliffs illuminated by strong sunlight ; even 

 the application of a microscope brought out no other feature than one 

 broad, faint, and nearly-uniform tint. But on applying the microscope 

 to photographs of distant hills taken at a high level in the atmosphere, 

 an abundance of minute detail appeared, and each little separate " re- 

 tama " bush could be distinguished on a hill-side four and a half miles 

 from the camera. Specimens of these photographs thus magnified 

 have been introduced in to the newly-published volume of the Edinburgh 

 Astronomical Observations. 



