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A, Clark on the Sun and Stars photometrically compared. 5 
the limit at which the naked human eye could ever perceive 
this great luminary. 
I have an under-ground, dark chamber, 230 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 verti- 
cal opening, one foot square, and five feet'deep. In a moveable 
partition, between this opening and the end of the chamber, a 
lens of such focal distance as I choose can be inserted. A twen- 
tieth of an inch focus I have employed, of the best finish possi- 
te its flat side cemented to one face of a prism with Canada 
alsam, 
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 o total re- 
flexion, by which they are directed through the little lens into 
the chamber. 
_ An observer, in the cellar, 230 feet distant, sees the sun re- 
duced 55,200 times; and its light, in amount, varies but little 
from that of Sirius. 
Upon a little car, moveable 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 perfectly clear sky, I found the 
sun visible through the six inch lens, when it was removed to 
e distance of 12 feet from the eye. The distance between the 
lenses being 218 feet, the reduction by the small lens, if viewed 
from the point occupied by the six inch lens, would be 52,320 
times; and that again by the six inch, distant from the eye 12 
cet, or 24 times its focal distance, is reduced 23 times; 
the total reduction 1,203,360 times. - ; 
7 mes now an important matter to ascertain as nearly as 
Possible the proportion of light lost, by and through the media 
_ above described; the looking-glass, the prism, and two lenses ; 
though joining the little lens with balsam to the prism, it may 
__ be regarded as one piece. 
| minimum visibile, does not exceed one-eighth part of the entire 
, and could not reach one-seventh, when the prism and 
