June 19, 1890] 



NATURE 



185 



but let us first analyze our diagrammatic moon — let us magnify it 

 about ten times, and see what it looks like. 



I now show you a picture of this part of the diagram, in- 

 closing the portions I wish to speak about, magnified ten times, 

 so that you can see that about twenty-eight of our points, and by 

 supposition twenty-eight of our particles of silver on the photo- 

 graphic plate, make up the picture. You will see that these dots 

 vary in size ; the difference is due to the amount of light falling 

 within what we may call the sphere of action of each point, and 

 should represent it exactly. The result can hardly be called 

 a picture, as it conveys no impression of continuity of form to 

 the mind. We have got down to the structure or separate parts, 

 and to the limit of the powers of the eye and the photographic 

 plate, of course on the assumption we have made as to the size 

 of the points in the one case and the particles of silver in the 

 other. I will now show you the same parts of the moon as re- 

 presented by the circles on our diagram exactly as delineated by 

 photography. You now see a beautiful picture giving mountains, 

 valleys, craters, peaks, and plains, and all that makes up a 

 picture of lunar scenery. We have thus seen how the power of 

 the eye is increased by the enlargement of the picture on the 

 retina by the telescope, and also how, by increasing the size of 

 the photograph, we also get more and more detail in the picture. 

 We know we cannot alter the number of those separate points 

 on the retina which determine the limit of our powers of vision 

 in one direction, but we may be able to increase enormously the 

 number of particles of silver in our photographic picture by 

 processes that will give finer deposits, and so, in conjunction 

 with more perfect and larger photographic lenses, we may 

 reasonably look for a great improvement in our sense of vision — 

 it may be even beyond that given by the telescope alone ; al- 

 though it always will be something in favour of the telescope 

 that the magnification obtained in the eye is about fifteen times 

 greater than that obtained by photography when the image on 

 the retina is pitted against the photograph of the same size, 

 unless we use a lens to magnify the photograph of the same focal 

 length as the eye, in which case it is equal. But we may go 

 much further in our magnification of the photographic image. 

 In other ways there is great promise when we consider the 

 difference in the action of the eye and the chemical action in 

 the sensitive film under the action of light. As I pointed out in 

 the discourse I gave about four years ago in this theatre, the 

 eye cannot perceive objects that are not sufficiently illuminated, 

 though this same amount of illumination will, by its cumulative 

 effect, make a photographic picture, so that there are ways in 

 which the photographic method of seeing celestial bodies can be 

 possibly made superior to the direct method of looking with a 

 telescope. 



With some celestial objects this has been already done : stars 

 too faint to be seen have been photographed, and nebulae that 

 cannot be seen have also been photographed ; but much more 

 than this is possible : we may be able to obtain photographs of 

 the surface of the moon similar to those I have shown, but on a 

 very much larger scale, and we may obtain pictures of the 

 planets that will far surpass the pictures we would see by the 

 telescope alone. 



I have mentioned that the distance at which the normal eye 

 can best see things is about nine inches, as that gives the greatest 

 angular size to the object while retaining a sharp picture on the 

 retina ; but, as many of us know, eyes differ in this power : two 

 of the common infirmities of the eyes are long- or short-sighted- 

 ness, due to the pictures being formed behind the retina, in the 

 first case, and in front of it in the other. Towards the end of the 

 thirteenth century it was found that convex lenses would cure the 

 first infirmity, and, soon afterwards, that concave lenses would 

 cure the second, as can be easily seen from what I have said 

 about the action of these lenses ; so that during the fifteenth and 

 sixteenth centuries the materials for the making of a telescope ex- 

 isted ; in fact, in the sixteenth century. Porta invented the camera 

 obscura, which is in one sense a telescope. It seems very strange 

 that the properties of a convex and concave lens when properly 

 arranged were not known much earlier than 1608. Most prob- 

 ably, if we may judge from the references made by some earlier 

 writers, this knowledge existed, but was not properly appreciated 

 by them. Undoubtedly, after the first telescopes were made in 

 Holland in 1608, the value of this unique instrument was fully 

 appreciated, and the news spread rapidly, for we find that in 

 the next year "Galileo had been appointed lecturer at Padua 

 for life, on account of a perspective like the one which was sent 

 from Flanders to Cardinal Borghese. " As far as can be ascer- 



NO. 1077, VOL. 42] 



tained, Galileo heard of the telescope as an instrument by which 

 distant objects appeared nearer and larger, and that he, with 

 this knowledge only, reinvented it. The Galilean telescope is 

 practically, though not theoretically, the simplest form. It is 

 made of a convex lens in combination with" a concave lens to 

 intercept the cone of rays before they come to a focus, and 

 render them parallel so that they can be utilized by the eye. 

 It presents objects as they appear, and the picture is freer from 

 colour in this form than in the other, where a convex eye-glass 

 is used. It is used as one form of opera-glass at the present 

 time. Made of one piece of glass in the shape of a cone, the 

 base of which is ground convex, and the apex slightly truncated 

 and ground concave, it becomes a single-lens telescope that can 

 be looked upon just as an enlargement of the outer lens of the eye. 



Galileo was undoubtedly the first to make an astronomical 

 discovery with the telescope : his name is, and always will be, 

 associated with the telescope on this account alone. 



Very soon after the introduction of the Galilean telescope, the 

 difficulties that arise from the coloured image produced by a single 

 lens turned attention to the possibility of making a telescope by 

 using the reflecting surface of a concave mirror instead of a lens. 

 Newton, who had imperfectly investigated the decomposition of 

 light produced by its refraction through a prism, was of opinion 

 that the reflecting principle gave the greatest possibilities of in- 

 crease of power. He invented, and was the first to make, a 

 reflecting telescope on the system that is in use to the present 

 day ; thus the two forms of telescope — the refracting and re- 

 flecting — came into use within about 60 years of each other. It 

 will be perhaps most convenient in briefly running through the 

 history of the telescope, that I should give what was done in 

 each century. 



Commencing, then, with the first application of the telescope 

 to the investigation of the heavenly bodies by Galileo in 1609, 

 we find that the largest telescope he could make gave only a 

 magnifying power of about 30. 



The first improvement made in the telescope, as left by Galileo^ 

 was due to a suggestion — by some attributed to Kepler, but 

 certainly used by Gascoigne — to replace the concave eye-lens 

 that Galileo used by a convex one. Simple as this change looks, 

 it makes an important, indeed vital improvement. The tele- 

 scope could now be used, by placing a system of lines or a scale 

 in the common focus of the two lenses, to measure the size of 

 the image produced by the large lens ; the axis or line of coUi- 

 mation could be found, and so the telescope could be used on 

 graduated instruments to measure the angular distance of various 

 objects ; in fact, we have now in every essential principle the 

 true astronomical telescope. It is useless as an ordinary tele- 

 scope, as it inverts the objects looked at, while the Galilean 

 retains them in their natural position. The addition, however, 

 of another lens or pair of lenses reinverts the image, and we 

 then have the ordinary telescope. It was soon found that the 

 single lens surrounds all bright objects with a fringe of colour, 

 always of a width of about one-fiftieth of the diameter of the 

 object-glass, as we must now call the large lens ; and as this 

 width of fringe was the same whatever the focal length of the 

 object-glass, the advantage of increasing this focal length and 

 so getting a larger image without increasing the size of the 

 coloured fringe became apparent, and the telescope therefore 

 was made longer and longer, till a length of over one hundred 

 feet was reached ; in fact, they were made so long that they 

 could not be used. A picture of one of these is shown, from, 

 which it can be easily imagined the difficulties of using it must 

 have been very great, yet some most important measurements 

 have been made with these long telescopes. Beyond the sug- 

 gestions of Gregory and Cassegrain for improvements in the 

 reflecting telescope, little was done with this instrument. 



During the eighteenth century immense advances were made 

 in both kinds of telescopes. With the invention of the achromatic 

 telescope by Hall and Dollond, the long-focussed telescopes 

 disappeared. 



Newton had turned to the reflecting telescopes believing from 

 his investigations that the dispersion and refraction were constant 

 for all substances ; this was found not to be so, and hence a 

 means was possible to render the coloured fringe that surrounds 

 bright objects when a single lens is used less prominent, by 

 using two kinds of glass for the lens, one giving more refraction 

 with somewhat similar dispersion, so that while the dispersion 

 of one lens is almost corrected or neutralized by the other, there 

 is still a refraction that enables the combination to be used as a 

 lens giving an image almost free from colour. 



