344 



THE POPULAE EDUCATOR 



KECREATIVE SCIENCE. VII. 



THE PRISM (A REFRACTING INSTRUMENT), AND DIS- 

 COVERIES MADE WITH THE SPECTROSCOPE. 



OF all the optical lenses the prism is the most important and 

 instructive, and it has enabled philosophers to add another 

 branch of science, called " spectrum analysis," to those already 

 known. With the help of this triangular piece of glass, we 

 are enabled to decompose and analyse a ray of light, and from 

 the knowledge so obtained to account for the cause of colour. 



If a ray of sunlight is allowed to pass through a hole half 

 an inch in diameter into a room, the walls of which should be 

 as dark as possible or hung with black calico, and a prism 

 intersect it, as at a (Fig. 1), the ray will cease to go forward in 

 the direction c d, but will be decomposed, and exhibit on a 

 white screen a beautiful spectrum, consisting of seven colours 

 red, orange, yellow, green, blue, indigo, and violet. With an 

 ordinary glass prism, such as those used for glass lustres, the 

 edges of the colours are not clearly defined, but seem to melt 

 or mix one into the other. If a hollow glass prism, filled with 

 bisulphide of carbon, is employed, the seven colours called the 

 " solar spectrum " are much more clearly defined. 



Sir Isaac Newton made the important discovery that white 

 light is a compound of rays of various kinds, having different 

 colours and indices of refraction ; and that all substances which 

 appear coloured when illuminated with white light derive their 

 colours only from a kind of "natural selection" i.e., they may 

 reflect certain coloured rays, and transmit or extinguish others. 

 Light is the fountain of all colour; but it does not follow, 

 because a substance known to have a particular colour is visible, 

 that the colour is inhorent. This will depend upon the nature 

 of the light used. By the combustion of thallium (a metal 

 discovered by Mr. Crookes), a most beautiful green light is 

 obtained ; and if this is directed on to a red substance, such 

 as a stick of red sealing-wax, the latter appears dark in fact, 

 black. Although there is plenty of light to show the object, 

 the light does not contain the red ray which is necessary for 

 the lighting up of the red substance. On the other hand, if a 

 Tom Thumb geranium is put into the light produced by the 

 thallium, the leaves appear to be most vividly groen, whilst the 

 red flower is dark. In this case, the flower or the red sealing- 

 wax completely absorbs or quenches the green light, and it is 

 only the green leaves that send back or reflect the green light. 

 From these experiments it must be apparent that coloured tex- 

 tures or pigments appear to be red, yellow, or green, or whatever 

 their colour may be, when white light falls upon them, because 

 light is the storehouse of colour ; and if a substance is red, it 

 must destroy the orange, the yellow, the green, the blue, the 

 indigo, and the violet rays, and having drank in the latter 

 colours, red only is thrown out from it, and so on with all the 

 Various tints ; or, what is perhaps more strictly correct, the 

 light coming from a red substance may still contain a certain 

 proportion of blue or yellow rays, but these are overpowered by 

 the predominating red ray. When the solar spectrum, obtained 

 as already described, is thrown on to a white screen, it is most 

 amusing to see the effect of the various coloured rays upon 

 different pigments, and if slips of coloured paper are used, the 

 results are very distinct. By passing the ray of white light 

 through two prisms (instead of one) filled with bisulphide of 

 carbon, the spectrum may be made to stretch much -further 

 across the screen, and the sunbeam undergoes by the second 

 refraction a greater amount of "dispersion." The colours are 

 now more decidedly separated, and the experiments with the 

 slips of coloured paper, or other pigments, can be made with 

 much greater facility. The drawing apart or separation of the 

 colours is called "dispersion;" and thus the spectrum may be 

 made shorter or longer by using prisms of different dispersive 

 power. Although it is difficult for the best-trained eyes to 

 point out the exact boundary of eaoh colour, Sir Isaac Newton 

 managed, by repeated experiments, to convince himself that the 

 lengths of the colours with the particular glass prism, he used 

 were as follows : Eed, 45 ; orange, 27 ; yellow, 40 ; green, 60 ; 

 blue, 60 ; indigo, 48 ; violet, 80. Total, 360. 



He also ascertained that the colours could be brought togethei 

 again, re-combined, and that the result was the re-composition 

 of white light. This synthesis of the colours is easily shown 

 by using a second prism in an inverted position, as shown by 

 the dotted lines A A A (Fig. 2), or by allowing the coloured 



rays to fall upon a double convex lens (A B, Fig. 3), when they 

 are brought to a focus at c, and a spot of white light alone is 

 visible. 



The experiment may be varied by mixing seven different 

 coloured powders together, the colours, of course, being those 

 of the solar spectrum ; or the colours may be painted on a 

 circular piece of cardboard, and when this is properly mounted, 

 and turned with sufficient velocity, the colours all blend to- 

 gether and produce the nearest imitation of white light. 



If a sunbeam is sent through a double convex lens, which 

 represents a series of prisms with their bases attached and 

 their thinnest edges outward, it is not surprising that the disc 

 of light obtained should be fringed with colours, because it 

 has been shown that a prism decomposes white light. 



If all the colours had the same refrangibility, there would be 

 no fringes of colour on the edges of bodies seen through a 

 common telescope or opera-glass ; but as the focus of the red 

 ray is formed further away from the lens than that of the blue 

 ray, because the latter is more refracted than the former, it 

 follows that a separation of colour must occur, which is called, 

 in technical language, chromatic aberration. 



In the annexed figure (Fig. 4), the focus of the blue rays is 

 shown at B, and that of the red rays at B ; but this difficulty 

 has been most ingeniously surmounted by combining lenses of 

 unequal dispersive material ; and it was Dollond who proved, in 

 1757, that by combining a concavo-convex lens of flint glass 

 with a double convex one of crown glass, a lens was obtained 

 which virtually refracts the various coloured rays to one focus, 

 and is therefore free from colour, or achromatic. 



For absolute achromatism various lenses are necessary, but 

 for all practical purposes two are found to be sufficient, pro- 

 vided their curvatures are such as to combine the yellow and 

 red rays. 



There was one feature in the solar spectrum which escaped 

 the notice of Sir Isaac Newton, and it only shows how much 

 knowledge may be lost by performing an experiment in the 

 least perfect manner. Newton allowed his sunbeam to pass 

 through a circular hole, and therefore missed the dark bands 

 and fixed lines which cross the colours from the red to the violet 

 end of the spectrum, at right angles to its length. 



Brewster thus describes the discovery, which appears to have 

 been chiefly due to the fact that Wollaston admitted the light 

 from a narrow slit instead of a circular aperture : 



" In the year 1802, Dr. Wollaston announced that in the 

 spectrum formed by a fine prism of flint glass, free from veins, 

 when the luminous object was a slit the twentieth of an inch 

 wide, and viewed at the distance of ten or twelve feet, there 

 were two fixed dark lines, one in the green and the other in the 

 blue space. This discovery did not excite any attention, and 

 was not followed out by its ingenious author." 



Without knowing of Dr. Wollaston's observations, the late 

 celebrated M. Fraunhofer, of Munich, by viewing through a 

 telescope the spectrum formed from a narrow line of solar light 

 by the finest prism of flint glass, discovered that the surface of 

 the spectrum was crossed throughout its whole length by dark 

 lines of different breadths. None of these lines coincide with 

 the boundaries of the coloured spaces. They are nearly 600 in 

 number ; the largest of them subtends an angle of from 5" to 

 10". From their distinctness, and the facility with which they 

 may be found, seven of these lines, viz., B, c, D, E, F, a, H, 

 have beem particularly distinguished by M. Fraunhofer (Fig. 5). 



A very pretty and most useful apparatus has been invented 

 by Mr. John Browning, called the " miniature spectroscope," by 

 which at any time the solar spectrum may be observed in all 

 its beauty of colour, and the dark lines are easily seen by pro- 

 perly adjusting the width of the slit. When this is widely 

 opened the spectrum is more brilliant, because more light is 

 admitted to the prisms contained in the instrument, but the 

 lines are not then visible. By reducing the size of the aperture, 

 it presents the appearance of a striped ribbon, and is found to 

 ba crossed in the direction of its breadth by a number of dark 

 lines. 



The instrument in its case measures four inches in length, 

 and rather more than three-quarters of an inch in diameter; it 

 is therefore easily carried in the pocket, and thus kept ready for 

 any special use, such, for instance, as observing the bright 

 bands of colour emitted by certain flames or intensely-hot 

 gaseous matter, similar to that coming from the furnace in 



