152 



ASTRONOMICAL PHENOMENA AND PROGRESS. 



through the two lenses when the movable one 

 was 12 feet from the eye, and when being 218 

 feet from the fixed lens, the reduction given by 

 the latter alone was 52,320 diameters, and the 

 multiplying effect of the second lens = 12 x 2 

 1 = 23 times, the total reduction being thus 

 1,203,360 times. By observations expressly 

 devised for such purpose, he concludes that the 

 proportion of the light of the sun or a star that 

 will be lost in these experiments by the extin- 

 guishing action of the mirror, prism, and lenses, 

 will be in effect almost exactly compensated 

 by the additional light also reflected by the 

 mirror from a small region of sky just about 

 the sun or star. Proceeding upon tkis admis- 

 sion, the following are the results at which, in 

 his earlier observations, Mr. Clark arrived ; 



The sun Is visible when reduced 1,200,000 times. 



ThefUllmoon 8,000 ' 



girius 20 ' 



Procyon 12 " 



Pollux 11 " 



Castor 10.8 " 



The following comparisons will show the re- 

 lation in which these results stand to the meas- 

 ures of the sun's light previously given by- Dr. 

 Wollaston, and by Mr. Bond, of Harvard Col- 

 lege Observatory. To reduce our sun to the 

 brightness of the star a Lyra?, the distance of 

 the former must be increased, according to 



TVollaston, nearly 425,000 times. 



Bond, tt 155,000 " 



Clark 102,000 " 



And the light received from these luminaries 

 differs, according to 



Wollaston. as... 180,000,000,000 to 1 



Bond, " 24,000.000,000 " 



Clark, " 10,400,000,000 " 



Mr. Clark's method, it will be seen, does not 

 depend upon comparisons with artificial lights, 

 but makes a simple reduction of the luminary 

 observed to a minimum visibile, under the most 

 favorable conditions of observation, the stand- 

 ard in all cases. 



In his later experiments, he prepared a close 

 covering for the opening to the dark chamber, 

 with a circular perforation, subtending at the 

 prism an angle of 32", and substituted a lens of 

 one-eighth-inch focal distance. Then, by use of 

 two additional lenses, adjustable by sliding, and 

 placed in a telescope tube properly darkened 

 within, he found that it required on some oc- 

 casions a reducing power of nearly 1,600,000 

 to send the sun completely out of sight. 



Mr. Clark shows that these observations have 

 nn important bearing on the question of the ex- 

 istence of an extinguishing medium in space. 

 The more powerful telescopes reveal, in pro- 

 portion to their power, a far less number of 

 stars than are visible to the unassisted eye ; in 

 other words, the appearance is as if the re- 

 moter fields of space were more and more 

 thinly tenanted with stars, in comparison with 

 the number within the sphere of direct vision. 

 This fact has been made an argument for an 

 extinguishing medium in space; the greater 

 sparseness of the more distant or telescopic 

 stars being supposed due to the circumstance 



of entire extinction of the light which would 

 otherwise reach us from the larger proportion 

 of the stars of those regions. But Mr. Clark 

 suggests, what is obviously true, that if differ- 

 ent stars actually differ in original or inherent 

 splendor, then it will be the least luminous 

 which at any given distance will first elude the 

 eye, and as the distance is increased, a contin- 



. ually larger proportion of all the stars will thus 

 as a simple effect of reduction by increasing 

 distance disappear ; so that the sparseness of 

 stars in the outermost yet penetrated regions 

 of the universe does not necessarily prove the 

 presence of an absorbing medium, or ether, be- 

 tween their place and the earth, but may mere- 

 ly illustrate the known and simple relation of 

 the apparent magnitude or brilliancy of a visi- 

 ble object to its distance. 



The Question of the Sun's Distance from the 

 Earth. Professor Joseph Levering, of Har- 

 vard College, has communicated to the "Amer- 

 ican Journal of Science " (Sept., 1863), a highly 

 important paper upon the subject of the sun's 

 distance from the earth, as computed from the 

 several sorts of data relied on, and especially 

 upon the remarkable variance of the result very 

 recently obtained by M. Foucault from previous 

 calculations, and the general effect of this va- 

 riance, if con'firmed, upon the distances and 

 magnitudes of the various astronomical bodies. 

 First, as to the usual methods for determin- 

 ing the sun's distance : To see the distance of 

 any body is an act of binocular vision. In case 

 of near bodies, the interval between the two eyes 

 is the base-line of a triangle of which straight 

 lines from the object to the eyes respectively 

 form the other two sides ; and the sensation of 

 effort in converging the eyes upon the object, 

 guided by experience, gives us approximately 

 the distance. As the object is farther removed, 

 the base-line must be taken greater, until, in 

 attempts to determine the distance of the sun, 

 it is made the distance between two telescopes 

 directed toward that body from points at the 



.opposite extremities of the earth's diameter; 

 and certain parts of the triangle, giving the 

 distance of the object, are now found by calcu- 

 lation. The angle between the directions of 

 the two telescopes is the ''solar parallax ; " and 

 the distance of the sun will vary the base-line 

 being supposed known as the magnitude of 

 this angle. Since a small error in the solar 

 parallax would involve a large error in the 

 sun's distance, astronomers select a planet com- 

 ing nearer the earth than the sun either Ve- 

 nus, at inferior conjunction, or Mars at opposi- 

 tion. The former observation can only be 

 made in case of a transit across the sun's disc, 

 the quantity determined bemg the difference 

 of parallax between Venus and the sun : viz., 

 from about 21" to 25". From the combined 

 observations of the two transits of Venus last 

 occurring 1761 and 1769 Encke deduced the 

 solar parallax as 8".57116. This corresponds 

 to a solar distance of 95,360,000 miles. Trans- 

 its of Venus will occur in 1874 and 1882 ; but 



