10 



KNOWLEDGE. 



[November 1, 1889. 



made. The gi-eat authority of Newton no doubt checked 

 experiment in this direction for a generation, and tlie mis- 

 take is much to be regretted. In his Ojitics, Book I., 

 Part n., Experiment 8. Newton says : — " I found, more- 

 over, that when light goes out of air through glass, and 

 thence goes out again into air, whether the refracting 

 superficies be parallel or inclined to one another, that 

 light as often as by contrary refractions 'tis so corrected 

 that it emergeth in Hues parallel to those in which it was 

 incident, continues ever after to be white. But if the 

 emergent rays be inclined to the incident, the whiteness 

 of the emergent Ught wiU by degrees in passing on fi-om 

 the place of emergence, becone tinged in its edges with 

 colom-s. This I try'd by refi-acting hght with prisms of 

 glass placed within a prismatic vessel of water." From 

 this Newton concluded that no combination of lenses of 

 different refi-active index would bring light to a focus vnth- 

 out separating dift'erent colours of the spectrum in the 

 same way as they are separated by a single lens. 



It was not till 1729, two years after the death of Newton, 

 that Mr. Chester Moor Hall, a yoimg barrister of the Inner 

 Temple, succeeded in combining two lenses of dili'erent 

 sorts of glass so that they gave colourless images. It is 

 known that he constructed several achromatic telescopes, 

 one of two and a half inches apertm'e and only twent}' 

 inches focal length, which was in existence in 1790. But 

 how he arrived at his invention, whether experimentally 

 or deductively from experiments with prisms, is not known. 

 No use of this new and important weapon for astronomical 

 discovery was made for more than a quarter of a century, 

 and the invention had been forgotten, or perhaps it was 

 not ever known to any very wide circle. At all events, it 

 cannot kave been known to the members of the Coimcil of 

 the Royal Society, who, thu-ty-two years after, in 1761, 

 awarded medals, and distributed the patronage of English 

 science. For they then gave the Copley l\Iedal to .John 

 DoUond for a similar discovery, of which he had seen the 

 commercial value, if not the astronomical use. 



.John DoUond was a Spitalfields weaver. He was born in 

 1706, and at the age of forty-six, having set up his son as an 

 optician and joined him in the trade, he commenced experi- 

 ments with prisms for the purpose of testing the truth of 

 the statement of Newton quoted above. Ha\ing found that 

 Newton was mistaken, he set himself to combine suitable 

 lenses of different refi-aetive index, and made an achro- 

 matic telescope. This is the story as given by DoUond 

 himself in a paper pubUshed in the I'ltil. Tram, of the 

 Royal Society. On the 19th of April 1H.5S, he took out a 

 patent for his invention, which eight years afterward was 

 disputed in a Coiu't of law ; for the patent had become 

 valuable, and other opticians, hearing of Moor HaU's dis- 

 covery, had begun to pirate it. Champness, an optician 

 in CornliiU, was proceeded against, and he brought work- 

 men into court who proved to the satisfaction of Lord 

 Mansfield that, twenty-five years pre^"iously, they had 

 made sinular instruments for Mr. iloor HaU. Lord 

 Mansfield upheld the patent, remarking that " It was not 

 for the person who locked up his mvention in his scrutoire 

 to profit by it, but he who brought it forth for the benefit 

 of the pubhc." 



I hope that it will not be thought that I wish to throw 

 any doubt on the genuineness of DoUond's discovery. I 

 believe this to be one of the very numerous duplicate dis- 

 coveries which may be quoted in the history of science. 

 Their existence is almost as remarkable as the number of 

 double stars, and the dupUcity of so many lines in the 

 spectrum — there must be some reason for it. It is not so 

 strange a phenomenon when we regard a discovery as the 

 last and most fiautful step in a chain of thoughts, many of 



wliich are the common property of the age in which the 

 discoverer lives. We are too apt to look at discoveries 

 as the sole production of one mind. That John 

 DoUond was an independent thinker and worker is, 

 I think, sufficiently proved by other tilings known about 

 him. He is said to have died from a fit of apoplexy, 

 brought on by intense appUcation in studying Clairaut's 

 Theory of the Moon. From the time of DoUond onward 

 the refracting telescope went on impro%ing ; but great 

 strides were made, towards the end of the century, in the 

 construction of refieeting telescopes, especiaUy at the 

 hands of Short and Sir WUliam Herschel. 



The reflecting telescope is, to a great extent, a 

 creation of English growth. It has been the favourite 

 instrument of English amateur astronomers. Sir WUham 

 Herschel, Sir Jolm Herschel, LasseU, the Earl of Rosse, 

 Common, and Isaac Roberts have all most successfuUy 

 used reflectors, and with the exception of Mr. Roberts 

 they have all made their own instruments. Reflecting 

 telescopes have not been received with such favom- abroad, 

 or even in America. With the exception of a 2b-inch 

 reflector, made by Dr. Henry Draper, no large reflectors 

 have been used in America, and very few on the Conti- 

 nent of Europe. The reflecting form of telescope has 

 been chosen by the gi-eat amateur astronomers (who have 

 done so much towards the progress of astronomy), pro- 

 bably because it is less costly to make than a refi'actor of 

 equal aperture, and there seems to be no limit to tlie size 

 of the reflector except the mechanical difficulties involved 

 in its construction, whereas with the refractor we are fast 

 approaching the practical hmit of size. If you double the 

 diameter and focal length of a refi'actor you quadruple the 

 amoimt of light that is lost by absorption ia the object 

 glass ; thus the thickness of the crown glass lens of the 

 great Washington 26-inch refractor at its centre is 1-88 

 inches, and the thickness of the flint - glass lens is 

 0-96 inches, making together a total thickness of glass 

 of 2-87 inches. Even with the clearest glass manu- 

 factured, more than half the light which falls on 

 such a thickness of glass is absorbed in passing 

 through it. If we double the thickness, more than tlu'ee- 

 quarters of the Ught would be absorbed, and less than one 

 quarter would be transmitted ; for the first half of the thick- 

 ness halves the light, and the second half halves it again. 

 The greatest loss of light is only for the centre of the ob- 

 ject glass ; but in aU parts the absoi-ption is quadrupled 

 for a lens of double aperture. It wUl thus be seen that 

 we are rapidly advancing towards the hmit where an 

 increase of size with the refractor wUl not give us an 

 increase of light. But with the reflector the whole of the 

 light reflected reaches the eye-piece, whatever is the size of 

 the instrimieut. The difficulties which set a limit to the 

 size of the reflector are chiefly mechanical. If the aper- 

 ture of an instrmuent is doubled, and the same propor- 

 tions are kept, the weight of each part of the instrument 

 is miUtipUed by eight. We have not yet approached the 

 proportions of the Eiffel Tower, or the Forth Bridge ; but 

 it is not to such immense increases of size that I look for 

 progress in our knowledge of the Universe. It is rather 

 to the increased sensitiveness of photographic plates, and 

 to improvements in the dii\ing, accurate figuring, and 

 mounting of telescopes. It has always been acknowledged 

 that the most important part of an instrument was the 

 man at the eye-piece end. And ia this day of large 

 instruments the relative importance of the man is becoming 

 gi-eater and greater. 



The great variety as weU as complication of the ap- 

 pUances which are now-a-days supplied to the Observer 

 by the Engineer may be judged of by our Frontispiece of 



