November 1, 1895.] 



KNOWLEDGE 



249 



Did space permit, many- more examples of adhesive 

 organs might be noticed, although the majority of the 

 most striking have beeu referred to. A\ hUe adhesive 

 mouths appear to be primitive features of the groups in 

 which they occur, other suckiug-organs, with the probable 

 exception of those of the cuttlefishes, seem to be com- 

 paratively late developments, and occur only among certain 

 members of the groups in which they are found at all. 

 All these appear to have beeu produced quite independently 

 in the various groups, and as a sucker is not capable of 

 indefinite variation in form, it is not to be wondered at 

 that structure found La one group is frequently paralleled 

 in a totally different group. We have, therefore, in these 

 organs another excellent example of that parallelism in 

 development to which reference has been so often made 

 in former articles in Knowledge. 



Note. — Figiu-e 1 ij taken from the "Studv of Mauimals" hj Sir 

 William Flower and K. Lvdekker, and Figures 2 and 4 from the 

 ■■ Study of Fishes '' bv Dr. txiinther, by courtesy of Messrs. Black. 

 Figure .3 is taken from Cooke's " ilolluscs," by courtesy of Messrs. 

 Macmillan. 



SPECTRUM ANALYSIS.-II. 



By J. J. Stewart, B.A.Cantab., B.Sc.Lond. 



BESIDES the method of obtaining a spectrum of a 

 source of light described in a former article," that 

 is, by means of refraction through a prism, there 

 is another method of great iuiportanoe which in 

 some respects is preferable to that of the ordinary 

 one in which the prism is used, and this is the method of 

 the diifraction grating. This method is not only of great 

 use in spectrum analysis, but is also valuable as being the 

 most accurate process for measuring the wave-length of 

 light. 



The phenomenon known as diffraction is an example of 

 the ability of light to bend round corners or sharp edges. 

 Light, as is well known, is propagated in straight lines, 

 and when it passes through an opening in a shutter and 

 falls on a screen a bright patch of light is seen on the 

 screen surrounded by a dark shadow ; the illuminated area 

 being bounded by a line or set of lines, which are obtained 

 by drawing straight lines from the boundary of the opening 

 to the boundary of the area lit up on the screen. The 

 sharply defined shadows cast by objects placed in direct 

 sunlight also indicate to everyone the fact of the rectilinear 

 propagation of light, and it is this rectilinear propagation 

 which formed the first great difliculty in the way of the 

 acceptance of the undulatory theory of light. This 

 difficulty has been completely got over, and the propa- 

 gation in straight lines has been explained by means of 

 the wave theory. 



When we come to consider the phenomena of diffi-action, 

 we have before us a set of facts which cannot be accounted 

 for by the old emission hypothesis of Newton, but have 

 been shown to follow naturally from the undulatory tlieory 

 of light. What these diffraction phenomena are I wiil 

 now shortly state. 



When light is allowed to pass through a small opening 

 — for example, a narrow slit with a lamp flame behind it — 

 a bright area is seen on a screen in front of the slit corre- 

 sponding in shape to the form of the slit. This bright area 

 is surrounded liy the darkness of the shadow, .and, roughly 

 speaking, the illuminated area consists of all those portions 

 of the screen to which straight lines can be drawn from 

 the source of light through the slit. But if the slit be a 

 narrow one, and we observe the effect on the screen 



.See KxowLEDGE, July, 1895. 



closely, it is seen that the shadow is not sharply defined ; 

 darkness does not commence at once at the edge of the 

 bright area, but a system of fringes is produced, coloured 

 bands are seen, and a succession of brighter and darker 

 areas parallel to the length of the opening through which 

 the light comes. If the light we use is of one colom- only — 

 if, for example, it passes through a piece of red glass before 

 it falls on the slit — a series of bright red bands alternating 

 with dark and completely black ones is noticed. The 

 distance apart of the bands depends partly on the width 

 of the slit and its distance from the screen, and they 

 become obliterated and fade into uniform darkness at a 

 short distance from the bright central area. If the 

 distance between the slit and the screen is not too great, 

 another set of fringes can be seen inside the bright area 

 which is the projection of the opening on the screen. 



SimUar effects are obtained when light is obstructed by 

 an opaque obstacle of narrow width, or when it passes by 

 the sharply-defined straight edge of some obstacle. It was 

 in this form that the phenomenon seems to have been first 

 observed about the middle of the seventeenth century, by 

 Grimaldi, but the explanation was not known till con- 

 siderably later. He allowed light to pass into a darkened 

 room through a very small round opening, and on placing 

 a small opaque obstacle in the path of the cone of light thus 

 produced, he noticed that its shadow on a screen was much 

 larger than its geometrical projection. This showed that 

 the path of the light was not altogether a straight line, 

 but that it was bent out of its rectilinear course owing to 

 the presence of the obstacle. The shadows he observed to 

 be bordered by three raiubow-colom-ed bands, the direction 

 of which was parallel to the edge of the shadow, while 

 their intensity fell off' as they became more distant from 

 the edge. Newton also noticed these effects, and he let 

 light pass through the narrow opening between two knife 

 edges, getting an image on a screen which was bordered 

 by three bright bands which were violet nearest the shadow 

 and red farthest away from it. 



These remarkable appearances are readily produced, and 

 can be seen by anyone who places a narrow slit cut in a 

 piece of cardboard near a candle flame, and then views it 

 at the distance of a few feet through another slit held 

 parallel to the first and close to the eye. The appearance 

 of the first slit as thus seen will be that of a bright band 

 of light surrounded on each side by a set of parallel bands 

 of alternate light and darkness, the brighter parts being of 

 various colours. 



In order to produce a spectrum by means of diffraction a 

 " grating " is used. This consists of a number of parallel 

 lines ruled on a glass plate, at equal distances apart, by 

 means of a diamond point. Light falling on such a plate 

 is reflected at the lines, but passes freely through the 

 spaces between. The diffraction grating has thus the same 

 effect as a system of fine opaque threads alternating with 

 clear spaces. We may also consider it as an arrangement 

 of numerous slits very close together. Gratings thus con- 

 structed often contain from twenty thousand to forty 

 thousand lines to the inch, and the rulings can only be 

 seen when the grating is looked at through a powerful 

 microscope. Diffraction spectra can also bo produced by 

 placing a piece of fine wire gauze in the course of the 

 light, the opaque wires taking the place of the rulings ; 

 but with a coarse arrangement like this the spectra are 

 not nearly so good, the dispersion and the purity being 

 less. The production of a good grating is a matter of 

 great difficulty. The diamond point is apt to break, and 

 it is not easy to make the rulings at equal distances 

 throughout the whole grating ; errors thi-ough variation 

 in the spacing would cause irregularities in the spectra 



