154 



KNOWLEDGE. 



[August 1, 1891. 



Fig. 2. — Shell or Ammo.vite, 



the free swimming forms, 

 locomotion is effected by the 

 forcible expulsion of a jet of 

 water from a funnel situated 

 near the head, and directed 

 forwards, the result of which 

 is to propel the animal forcibly 

 backwards. Minor aid in 

 swimmmg is afforded either 

 by expansions of the skin on 

 the sides of the body, or by 

 distinct fins near the tail ; 

 while many of these creatures 

 aid their escape from foes by 

 the sudden discharge of an 

 inky fluid during their back- 

 ward course. 



Our notice of Swimming 

 Invertebrates cannot be con- 

 cluded without mention of those curious marine animals 

 known as Sea-Squirts, and technically as Tunicates— a 

 group usually placed in the neighbourhood of the Molluscs. 

 Although in the adult state many of the Tunicates exist in 

 the form of the bag-like squirts with which many of us are 

 familiar, yet all are free-swimming creatures in the young 

 condition. Moreover, certain of them, like the Salp;?, 

 are pelagic throughout their existence ; some of the latter 

 forming chains composed of numerous individuals attached 

 to one another. These SalpiC-chains vary in length, from 

 a few inches to several feet, and swim on the ocean surface 

 with a serpentine movement. The great interest attaching 

 to these Timicates is that the yoimg exhibit certain 

 structures closely simulating the primitive condition of the 

 spinal column of Vertebrates, and thus suggesting that 

 they are degraded tyj^es allied to the original stock from 

 which the Vertebrates themselves are descended. This is 

 very important as regards the derivation of Vertebrates 

 from aquatic animals ; — an origin which we should 

 naturally expect, seeing that fishes breathe by means of 

 gills, and are, therefore, presumed to have had aquatic 

 ancestors. 



[To he contimml.) 



ON THE SPACE-PENETRATING POWER OF 

 LARGE TELESCOPES. 



, By A. C. Eanyard. 



UNLESS there is some small star or dimly shining 

 body with a large parallax which has not yet 

 been detected, our nearest neighbour amongst 

 the stars is the double star a Cfntmiri. It is 

 situated about thirty degrees 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 1st magnitude 

 star, for the larger of these twin suns is ranked by 

 Prof. Gould as being exactly of the 1st magnitude of 

 the photometric scale, and the smaller star is of the 

 3i magnitude. 



According to this photometric scale of magnitudes, which 

 is now universally used, a star of the 1st magnitude gives 

 just 100 times as much light as a star of the 6th magnitude. 

 Consequently, if the larger star of the pair, which is known 

 as a- Centauri, were removed to ten times its present 

 ilistance, it would appear as a star of the 6th magnitude ; 

 but this would only be the case if there were no loss of 

 Ught in travellmg from its more distant position. If there 



were any absorption of light in passing through such a vast 

 distance of space, it might appear smaOer and would 

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

 stars with their unaided eyes which are ranked as smaller 

 than the 6th 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 6th magnitude gives 2-512 

 times as much light as a star of the 7th magnitude, and a 

 star of the 7th magnitude gives 2-512 times as much light 

 as a star of the Kth magnitude. Consequently a star of 

 the 6th magnitude gives 6-31 times as much light as a star 

 of the 8th magnitude, and 15-85 times as much light as a 

 star of the 9th magnitude, 39-81 times as much light as a 

 star of the 10th magnitude, and 100 times as much light 

 as a star of the 11th magnitude. 



Let us suppose that a- Ccntnuri was removed to 100 times 

 its present distance, then, neglecting the absorption of 

 light in space, it would shine as a star of the 11th 

 magnitude of the photometric scale, and would only just 

 be visible with a telescope of two and a half inches aper- 

 ture. This calculation is based on the assumption of Prof. 

 C. A. Young'- that, for normal eyes, with a good telescope, 

 the •minimum rinhle for a one-inch aperture is a star 

 of the 9th magnitude — an estimate which about cor- 

 responds to what might be expected fr-om the diameter 

 of the pupil of the eye. 



I have measured the diameter of the pupils of several 

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

 the observing eyes of the Eev. 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 pupU in \'iewing faint objects. A telescope 

 of one inch diameter would, consequently, collect about 

 sixteen times as much light as would enter the pupil of the 

 unassisted eye, and ought, mth a suitable eye-piece, to show 

 stars gi^"ing about Y^th the light of a 6th magnitude star 

 just \-isible to the naked eye. As we have seen above, a 

 6th magnitude star gives 15-85 times as much light as a 

 0th magnitude star of the photometric scale. Consequently, 

 neglecting the absorption of hght 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 pupil of the eye the whole of the light from the 

 object-glass), to render stars of the 9th magnitude just 

 visible. 



The j)ower used with a telescope makes some difference, 

 as it increases the contrast between the brightness of the 

 star and the backgroimd on which it is seen — the light of 

 the background being dimmed by magnification, while the 

 star in a good defining telescope is but slightly dimmed by 

 moderate magnification. Thus Dawes found that he could 

 see a star of the 6th magnitude with a telescope having an 

 aperture of only 0-15 inches when a power of 16J- 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 seems to be about balanced by the 

 increase of contrast with the background, due to the 

 power employed. 



Let us suppose that a- Centnuri were removed to a 

 thousand times its present distance, then, neglecting the 

 absorption of light in travelling through space, it would 

 appear as a star of the 16th magnitude, and would only 

 just be visible with a telescope of 25-12 inches aperture, 

 and if it were removed to 1585- times its present distance, it 

 would shine as a star of the 17th magnitude of the photo- 

 metric scale, and would only just be visible in a telescope 



See Prof. Young's Text Book of General Astronomy, sec. 822. 



