HARDWICKE'S SCIENCE-GOSSIP. 



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have an elevating effect on the minds of all who 

 pursue this course aright, whatever branch of science 

 is taken up. Astronomy presents us with a view of 

 the workings of nature on the grandest scale ; he who 

 can survey the heavens on a clear night without 

 some noble thoughts entering his breast must indeed 

 be earthly. The more one studies the heavens, the 

 greater must be one's admiration of the works of the 

 Great Architect of the Universe. 



An astronomical observatory may be briefly denned 

 to be a place where astronomers observe the heavens 

 by means of special • instruments. I purpose to 

 describe somewhat minutely such a place, and the 

 various instruments and appliances to be found there, 

 as well as some of the wonders of the heavens as 

 seen by their aid, and to note the more important 

 discoveries that have been made at various times in 

 connection with the science of astronomy. 



The chief instrument employed is, of course, a 

 telescope, that is, a device by which distant objects 

 may be brought apparently nearer to us, so that we 

 may be enabled to scrutinise them with greater 

 minuteness. Most people are acquainted with the 

 outward appearance of a telescope, but its interior 

 arrangements are probably a mystery to them. It 

 may, therefore, not be lost time if we briefly consider 

 its history and examine its construction. There are 

 two sorts of telescopes, the one called refracting, the 

 other reflecting. Of these the older form is the 

 refractor. The credit of its discovery seems to lie 

 between three persons, named respectively Henry 

 Lipperhey, Zacharias Jansen, and James Metius, who 

 lived in the very early years of the seventeenth 

 century. One or other of these accidentally noticed 

 that by holding a concave glass near the eye, and at 

 the same time placing a convex one farther off, but 

 parallel to the first, distant objects^ appeared to be 

 magnified. Galileo, who had heard of this remarkable 

 discovery, constructed a somewhat similar instrument 

 in 1609. Two years later Kepler described how a 

 telescope could be formed by a combination of two 

 convex lenses, but he did not practically prove his 

 theory. Indeed it was not till about fifty years 

 afterwards that such instruments were employed 

 generally in making astronomical observations. The 

 early telescopes were of very small diameter, but 

 astronomers are always asking for " more light." It 

 must be remembered that the light received by an 

 object-glass, as the larger glass at that end of the tube 

 farthest away from the eye is called, when compared 

 with that received by another, varies as the squares 

 of their respective diameters. In the many attempts 

 that were made at different times to keep pace with 

 the demand for larger instruments, some important 

 drawbacks presented themselves. One was the great 

 difficulty experienced in making the lenses of the 

 proper shape ; for the proper curvatures of their 

 surfaces have to be determined mathematically, and 

 the glasses ground to these curves and then polished. 



Then, again, when object-glasses of larger size were 

 attempted (though this difficulty did not at first show 

 itself), it was found almost impossible to obtain a 

 glass disc that was homogeneous and free from streaks 

 and other imperfections. A third difficulty was that 

 met with in consequence of the phenomenon known as 

 chromatic aberration. If we take an ordinary lens 

 and look through it at a bright object, we shall find 

 that all the outlines of the latter are fringed with 

 different tints, commonly called the colours of the 

 rainbow. Such an array of colours round a celestial 

 object would naturally be a serious obstacle to any 

 accurate observation. It was not till 1729 that the 

 difficulty of producing glasses which should show an 

 image free from extraneous colour was surmounted 

 by Chester More Hall. Though he made several 

 instruments of this description he did not publish his 

 researches, and the credit of being the first to make 

 an achromatic lens, that is, one which will show 

 images of the objects viewed without the coloured 

 fringes, is assigned to John Dollond, thirty-one years 

 later. From his experimental researches on light, 

 an account of which will be found in the " Philo- 

 sophical Transactions " of the Royal Society for the 

 year 1758, Dollond found that the desired end could 

 be gained by placing together two lenses, the one 

 nearest the object to be viewed being made of crown 

 glass, and having its surfaces convex, the other made 

 of flint glass and with concave surfaces. Many 

 years, however, before the production of Dollond's 

 object-glasses, attempts were made to overcome the 

 difficulties we have mentioned by using telescopes of 

 a totally different construction. Instead of viewing 

 the object through a lens or lenses, the light pro- 

 ceeding from it was allowed to fall on a polished 

 mirror, made of metal and ground to a particular 

 shape. If we place a looking-glass against the wall 

 in the middle of a room, and, standing at one end of 

 the room, look in the glass, we shall see reflected 

 from it the objects that are at the other end of the 

 apartment. If now we carefully measure the angle 

 made by the surface of the glass and a line drawn 

 from those objects to that part of the glass where 

 they are seen, and that made by the same surface 

 and another line drawn from our eye to the same 

 point, we shall find these angles to be equal. To 

 put it into technical language, the angle of incidence 

 (that is, the angle at which the light falls) is equal 

 to the angle of reflection. The same law holds 

 good with all mirrors, whether their surfaces are 

 flat or curved. Consequently it was found that, by 

 using curved mirrors, the light falling on such a 

 one could be caused to be reflected in a direction 

 not far removed from that of the line in which it 

 originally fell. By using mirrors of proper curvature, 

 all the light will be reflected to one point ; whereas 

 in the case of a lens the light which falls on it is 

 i-efr acted, or bent to one point. Hence arise the 

 two names, reflector and refractor, as applied to 



