NATURAL PHILOSOPHY. 



97 



ON THE COMPARATIVE LIGHT OF THE SUN AND STARS. 



The following interesting research is communicated to Silliman's 

 Journal by Mr. Alvan Clark, the discoverer of the companion to Sirius : 



" If we place a lens of known focal distance, one foot for instance, 

 between the eye and a star of the first magnitude, or one of any con- 

 siderable brightness, with conveniences for guiding its movement in 

 distance, to any point where it may be needed, and find the star just 

 visible, or reduced to a sixth magnitude, when the lens, if a convex, is 

 eleven feet from the eye, it becomes clear that, since the star has un- 

 dergone a reduction of ten diameters, it would be visible, if removed in 

 space to ten times its present distance. This, however, is on the sup- 

 position that no absorbing or extinguishing medium exists in space. If 

 a concave lens be employed, the measure must be commenced at the 

 lens itself, but if convex, at the focal point ; or once the focal distance 

 must be subtracted from the measure, and the number of focal distan- 

 ces remaining corresponds to the number of reductions under which the 

 object is viewed. 



Castor is visible, when reduced, 



Pollux, 



Procyon, 



Sirius, 



The full Moon, . 



The Sun, . 



10-3 times. 



11 " 



12 " 



20 " 

 3,000 " 

 1,200,000 " 



I have actually seen the sun under such a reduction ; attended by 

 circumstances which have led me to believe that it is about the limit 

 at which the naked human eye could ever perceive this great 

 luminary. 



I have an under-ground, dark chamber, two hundred and thirty feet 

 in length, one end terminating in the cellar of my work-shop, and the 

 other communicating with the surface of the ground by a vertical open- 

 ing, one foot square, and five feet deep. In a movable partition, be- 

 tween this opening and the end of the chamber, a lens of such focal 

 distance as I choose can be inserted. A twentieth of an inch focus I 

 have employed, of the best finish possible ; its flat side cemented to one 

 face of a prism with Canada balsam. No light whatever can enter the 

 dark chamber, except through this little lens. A common, plane, silvered 

 glass mirror, placed above-ground, over the vertical opening, receives the 

 direct rays from the sun, and sends them down into the prism of total 

 reflection, by which they are directed through the little lens into the 

 chamber. An observer, in the cellar, two hundred and thirty feet dis- 

 tant, sees the sun reduced 55,200 times ; and its light, in amount, 

 varies but little from that of Sirius. Upon a little car, movable in 

 either direction, by cords and a pulley, is mounted another lens, with a 

 focal distance of six inches. The eye of the observer is brought into a 

 line with the lenses, or so near it, that he sees the light through the six 

 inch lens ; then, by the cord, he sends the car into the chamber, to the 

 greatest distance at which he can see the light, like that from a star of 

 the sixth or seventh magnitude. At noon, March 19th, with a per- 

 fectly clear sky, I found the sun visible through the six-inch lens, when 

 it was removed to the distance of twelve feet from the eye. The dis- 

 tance between the lenses being two hundred and eighteen feet, the re- 

 duction by the small lens, if viewed from the point occupied by the six- 



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