OF CURRENT INTEREST

Taranaki Daily News, 17 October 1931, Page 13 (Supplement)

OF CURRENT INTEREST

STARS AND BLIND SPOT

('By

Rev. B. Dudley, 'F.R.A.S.)

Stars and the Blind Spot. It has recently been pointed out by an amateur astronomer who would amuse himself and his friends that it is not without interest to centre a bright star on the blind spot, and watch it . disappear. As 'frequently stated in text-books on physiology, where the optic nerve leaves the retina at the back of the eye it necessarily takes up space which would otherwise be occupied by sensitive rods find, cones, ct-ncl ‘thus Icnvcs n non-senisi-tivo "blind spot.” Most people know that if one eye is closed and the other employed to look at a mark on a sheet of paper held about six inches in front of the face, another mark, located about three inches away from the first on the same sheet, will disappear. This is due to the image falling upon a blind spot. , “Since the sky cannot be moved back and forth like the sheet of paper,” says the astronomer alluded to, “the trick is to find a spot at the correct angular distance from a star. From the average eye, this seems, to be about 16 degrees. 'Take Vega, for example, and, having closed the left eye, use the right eye to look at a point about 16 degrees to the left of Vega (Albu'eo serves well). 'With a 1 little practice, the proper point will be found where Vega will disappear entirely from the diverted vision of the right eye. The effect is entertainingly startling.” New Zealand observers might at the present time of the year test the matter by facing the north at about 8 pan. and experimenting with the 'bright star Altair, in the constellation Aquila, and the Delphinus cluster, which is about 16 degrees to the east of it. locus the open 'left eye on the little group Delphinus according to the conditions given and. without altering the focus, experiment with averted vision until Altair is on the blind spot, and thus temporarily invisible. . Or, looking in a southerly direction, one of the stars in the pointers and one in the Southern Cross'might be chosen for the purpose. The writer noticed this optical effect 'by accident many years ago when comparing the brightness of neighbouring stars. (It should’ be stated perhaps that if the observations be taken with the right eye, that organ must be. focussed upon Altair, when the Delphinus trroup will become invisible. The reason for this is that the two blind spots are nearer each other than ate the centres of the eye-balls. It requires averted vision to turn the eye-'ball round so that the blind spot is directed toward the star.)

Excessive Cold of Meteorites. _ Picked up immediately after their fall to earth, before they have parted with 'heat acquired by. friction with the atmosphere, meteorites are often so hot that it is impossible to hold ■them in the hand. Quite frequently, however, bodies are intensely cold when found—very much colder in fact than neighbouring stones. The following examples may be interesting. A fall occurred in the Kangra district, Punjab, India, at 2.15 on the afternoon of July 14, 1'8'66. Its extreme coldness as compared with normal terrestrial rocks 1 is accepted as the typical illustration of meteorites found cold. Recently a good deal of information concerning these objects has been collected and some quotations are here given, together with the surname of the authority:— 'I (a) “The cold of the fragments that fell was so intense as to benum'b t'he hands of the coolies who picked them up, and who were obliged in consequence of their coldness instantly io drop them” (Houghton, I'Sub). (<b) “It appears that soon after their fall, the stones were intensely cold” (Kirkwood, 1867). (c) “It is recorded that one of the lar< T e fragments of the Dhurmsala (India) meteorite, which fell in I'B6o, was found iu moist earth half an houi 01, so after the falll, coated with' ica” (Young, 1893). . , (d) “The Dhurmsala stone is said to have been intensely cold when picked up -5 (Oliver, 1925). (e) “The Dhurmsala stone of 1860 is stated to have been intensely cold when picked up immediately after falL ing, frost forming on its surface” (Merrill, 1928). 'Some doubt lias been cast upon these records as not being sufficiently authentic. (But this is not the case with respect to a mcteoritic fall _ which took place at Colby, Wisconsin, in July, 1917. Dr. 0. C. Farrington, who was keenly interested in the matter and anxious to satisfy himself as to the real facts, made a close personal enquiry into the circumstances .at Colby. After interviewing the men who dug up the 'Stone, he was perfectly convinced that it was not only intensely cold, but that frost had actually settled on some of the pieces.

What Might Have Been. . In a chapter on "Where Does the Universe End? Dr. Oswald Thomas, in liis book, The Heavens and the Universe, calls attention to the fact that we might never have found out that the earth is spherical in shape. 'But for the comparative smallness of its size we could easily have missed this truth, and thus still have been under many delusions. For many thousands of years men believed the Earth to be flat. “And,” writes the doctor, “if the globe had been, let us say, ten thousand times as large as it is, while the conditions of life remained unchanged, it is possible, that the idea that the Earth must be round would never have occurred, to ns. Of course, from the appearance of t'he stars, which hav e ■taught us so many things, and have even revealed to us the shape of our Earth, men of exceptional faculties might have been deduced that the Earth is a sphere. But we imagine not only that our Earth might have been ten thousand times larger than it is, but that a peipetual Loudon fog might have lain over the whole surface of the Earth since the first beginnings of life. Under such conditions it is quite conceivable that wc should still be speaking of ‘the terrestrial disc. 5 Tim ‘curvature of the Earth’s surface’ would be an impossible phrase.” "While it may appear Io some readers futile to picture in this way what might or might not have been, such exercise of the gift of imagination is often of great value inasmuch as it helps us to realise that wc may be quite wrong in judging the contents and nature of the universe from our own terrestrial standpoint. Certainly our “astronomy” would have been a very different thing from what it is to-day were the Earth’s atmosphere as cloud-laden as is that of the planet Jupiter., In such case, we could never have seen the stars at all, and should Jiaye begs huagiug many; felsa idga.3,