SCIENCE 



NEW YORK, AUGUST 14, U 



THE SPACE-PENETRATING POWER OF LARGE 

 TELESCOPES.' 



UiTLEsa there is. some small star or dimly shining- body 

 with a large parallax which has not yet been detected, our 

 nearest neighbor amongst the stars is the double star a Cen- 

 tauri. It is situated about thirty d^rees from the southern 

 pole of the heavens, and therefore is not visible in England. 

 The two stars together shine with a light which is a little 

 greater than that of a first magnitude star, for the larger of 

 these twin suns is ranked by Professor Gould as being ex- 

 actly of the first magnitude of the photometric scale, and the 

 smaller star is of the 3* magnitude. 



According to this photometric scale of magnitudes, which 

 is now universally used, a star of the first magnitude gives 

 just a hundred times as much light as a star of the sixth 

 magnitude. Consequently, if the larger star of the pair, 

 which is known as a" Centauri, were removed to ten times 

 its present distance, it would appear as a star of the sixth 

 magnitude; but this would only be the case if there were no 

 loss of light in travelling from its more distant position. If 

 there were any absorption of light in passing through such 

 a vast distance of space it might appear smaller, and would 

 probably not be visible to the naked eye, for few people see 

 ■^tars with their unaided eyes which are ranked as smaller 

 than the sixth magnitude. According to the photometric 

 scale, a star of any magnitude gives about two and a half 

 times as much light as a star of the magnitude immediately 

 below it. Thus a star of the sixth magnitude gives 2.512 

 times as much light as a star of the seventh magnitude, 

 and a star of the seventh magnitude gives 2.512 times as 

 much light as a star of the eighth magnitude. Consequently 

 a star of the sixth magnitude gives 6.31 times as much light 

 as a star of the eighth magnitude, and 15.85 times as much 

 light as a star of the ninth magnitude, 39.81 times as much 

 light as a star of the tenth magnitude, and 100 times as 

 much light as a star of the eleventh magnitude. 



Let us suppose that a' Centauri was removed to one hun- 

 dred times its present distance, then, neglecting the absorp- 

 tion of light in space, it would shine as a star of the eleventh 

 magnitude of the photometric scale, and would only just be 

 visible with a telescope of two and a half inches aperture. 

 This calculation is based on the assumption of Professor C. 

 A. Young (Text Book of General Astronomy, sec. 822) that, 

 for normal eyes, with a good telescope, the minimum visible 

 for a one-inch aperture is a star of the ninth magnitude — 

 an estimate which about corresponds to what might be ex- 

 pected from the diameter of the pupil of the eye. 



I have measured the diameter of the pupils of several per- 

 sons whom I believed to have keen sight, amongst others, 

 the observing eyes of the Rev. T. W. Webb, Mr. Burnham, 

 and the late Dr. H. Draper, and have found that about a 

 quarter of an inch generally corresponds to the maximum 

 dilation of the pupil in viewing faint objects. A telescope 



' A. C. Ranyard, in Knowledge for August. 



of one inch diameter would consequently collect about six- 

 teen times as much light as would enter the pupil of the un- 

 assisled eye, and ought, with a suitable eye piece, to show 

 stars giving about one-sixteenth the light of a sixth magni- 

 tude star just visible to the naked eye. As we have seen 

 above, a sixth magnitude star gives 15.85 times as much light 

 as a ninth magnitude star of the photometric scale. Conse- 

 quently, neglecting the absorption of light by the lenses, and 

 the reflection from their surfaces, a one-inch telescope ought, 

 with a suitable eye-piece (which collects and sends into the 

 ptipil of the eye the whole of the light from the object-glass), 

 to render stars of the ninth magnitude just visible. 



The power used with a telescope makes some difference, as 

 it increases the contrast between the brightness of the star 

 and the background on which it is seen, — the light of the 

 background being dimmed by magniBcation, while the star 

 in a good defining telescope is but slightly dimmed by mod- 

 erate magnification. Thus Dawes found that he could see a 

 star of the sixth magnitude with a telescope having an aper- 

 ture of only 0.15 of an inch when a power of 16^ was used. 

 In the case of the one-inch telescope above referred to, the loss 

 of light by absorption and reflection at the surfaces of the 

 lenses seem to be about balanced by the increase of contrast 

 with the background, due to the power employed. 



Let us suppose that a' Centauri were removed to a thou- 

 sand times its present distance, then, neglecting the ab- 

 sorption of light in travelling through space, it would appear 

 as a star of the sixteenth magnitude, and would only just be 

 visible with a telescope of 25.12 inches aperture; and if it 

 were removed to 1,585 times its present distance, it would shine 

 as a star of the seventeenth magnitude of the photometric 

 scale, and would only just be visible in a telescope of 39.81 

 inches aperture. That is, it would not be visible in the great 

 Lick 36-inch refractor. 



These calculations are based on the assumption that there 

 is no absorption of light in passing through great distances 

 of space, and also on the assumption that there is no loss of 

 light in passing through such thick lenses. The thickness 

 of the object-glass of the Washington 26-inch refractor at its 

 centre is nearly three inches ; thus, the flint glass lens is 

 there 0.96 of an inch thick, while the crown glass lens is 

 1.88 inches thick at its centre. Such a thickness more than 

 halves the intensity of the emergent pencil; and the loss of 

 light by absorption in passing through the glass near the 

 centre of the Lick object-glass must be considerable. Exact 

 measures of the absorption of light by such great lenses wou^d 

 be of much interest. We may, however, probably assume 

 with some confidence, that if a" Centauri were removed to 

 twelve hundred times its present distance it would not be 

 visible in the Lick telescope, even though there were no ab- 

 sorption of light in space; and a" Centauri is probably larger 

 and brighter than our sun. (Assuming, with Mr. Gore, a 

 period of 77 years for this binary, and a parallax of .75 of a 

 second, the sum of the masses of the components will be 2.14 

 times the mass of the sun.) 



Stai-s smaller than our sun would be lost to sight at 

 smaller distances. Consequently the Milky Way must 

 either be nearer to us than a thousand times the distance of 



