244 



HARDWICKE'S SCIENCE-GOSSIP. 



extends previous to trial. To appreciate it, let the 

 reader look alternately full at a star of the fifth 

 magnitude, and beside it; or choose two, equally 

 bright, and about 3° or 4' apart, and look full at one 

 of them ; the probability is he will see only the 

 other — such, at least, is my own case." And I can 

 add, mine also. I have frequently put the state- 

 ment of Sir John Herschel to the test in other ways. 

 Thus, in looking for a boat or buoy in the water on 

 a dark cloudy night, when the object is barely visible, 

 I have often found it when I was not looking full at 

 it, and have convinced myself that I had discovered 

 the object, and proved this theory by looking care- 

 lessly aside a few degrees (more, generally, than 

 Sir John Herschel gives), when it became quite 

 evident 



One notable and melancholy occasion I well 

 remember. It was a December night on the 

 Atlantic. The cry of " Man overboard ! " had 

 aroused me, and hastening on deck, I found the life- 

 boat already manned and in the water, quickly going 

 astern, the patent life-buoy let go, and the engines 

 stopped. Soon an excited group had assembled on 

 the poop, and, curiosity satisfied, it soon became a 

 silent, watching group, that heeded not the passing 

 time as we rose and fell on the still, deep, gloomy 

 ocean. Opera-glasses and telescopes scanned the 

 dim horizon and the dark waters. " Do you see the 

 boat yet ? " " I think I see her." " How far off ? " 

 — and then the hail, " Have you found him ? " and 

 the dismal response, " No ! " 



I was not the first, or the second either, to see 

 the returning boat, but when I did I was much sur- 

 prised that I had not seen her sooner. I believe I 

 was looking intently at her in the direction indi- 

 cated by many who had better sight than mine, and 

 that it was when my eyes were diverted from the 

 object that it broke suddenly into view; when, 

 according to Sir J. Herschel, its image was thrown 

 on a portion of the retina more sensible to feeble 

 impressions than the centre of the area of vision. 

 I verified the fact by gazing at the boat as she rode 

 over the waves, when I found that she appeared less 

 plainly than when I was looking aside from her. 



Now, all these cases are of a similar nature. And 

 it must be distinctly remembered that the rule 

 applies only to feeble impressions of light, or dusk, 

 iu the night, or again, as "E. T. S." remarks, 

 " when the fading light is somewhat confusing." 



In explanation of the subject, I must not forget 

 to mention that Hueck states that, without altering 

 the direction of the axis of his eyes or the quantity 

 of light admitted, but merely by fixing his attention 

 on a side object, he was able to widen his pupils as 

 much as one -half more than their former diameter. 



The application of this (if it be generally true, 

 which I have failed to demonstrate very satisfac- 

 torily) to the point in question may be only a 

 reduction to Sir John Herschel's theory. 



Anatomists and physiologists have discovered 

 that there is one spot on the retina which is abso- 

 lutely insensible to light ; and if the image of any 

 object fall on this particular spot, the impression is 

 not conveyed to the brain— we do not seethe object. 

 Any person may prove this in a very simple manner, 

 by an experiment known, I believe, as " Mar- 

 riotte's," and which is generally allowed to be a 

 demonstration of the insensibility of that particular 

 part of the retina where the arteria centralis retina; 

 enters it. The spot is not in the axis of vision, but 

 internal to it — nearer the nose. The knowledge, 

 therefore, of its existence is no help in replying to 

 the Note-and-Query of " E. T. S.," but, in connec- 

 tion with the subject of retinal variable sensibility, 

 it may be interesting. I have often amused my 

 friends by the experiment, and, as it is also 

 instructive, I will describe it. 



Bring your thumbs together, touching by their 

 inner margins, the fingers closed on the palms of the 

 hands; maintaining them in this position, extend 

 your arms horizontally from you, keep your arms 

 and hands thus, steady, thumbs well upright, close 

 the left eye ; fix the right on the left thumb. You 

 now see the backs of your thumbs with the right 

 eye. Whilst keeping it fixed on the left thumb, 

 move the right outwards from its fellow very slowly. 

 Although the right eye is rigidly fixed on the left 

 thumb, you perceive the right moving outwards, 

 until it arrives at a certain point (about six inches 

 from the left), when you lose sight of it. Move it 

 still farther outwards, or upwards, or downwards, 

 and it comes into view ; but there is one position or 

 place in which it disappears altogether, because its 

 image then falls on a part of the retina which is 

 insensible to light. The same experiment may, of 

 course, be performed with the other eye. 



Barbadoes. J. P. M. Boileatt, M.B. 



NOTES ON THE FAUNA OE BRITTANY. 



PIERIS DAPLIDICE (the Bath-white) was 

 moderately plentiful along the coast. I took 

 the two first I saw with my hat, as they are weak 

 flyers. The best spots for them were pulches of a 

 yellow-flowered plant of the Cabbage order. The 

 same plant, I think, grows plentifully on our town- 

 walls round Southampton. I only met with one 

 female Bath-white. 



Colias Hyale and Edusa (the pale and dark 

 clouded yellows) were very plentiful in the lucern 

 fields. Hyale was rather the commoner of the two. 



Argynnis Dia (the Gorless fritillary) frequented 

 the small patches of heathery ground along the 

 coast, and with it were quantities of all our common 

 August butterflies. Argynnis Did is a reputed 

 British species. 



Among the moths I took a handsome and marked 



