64 



UNDULATORY FORCES. LIG 11 T. 



[MONO-CHKOMATISM. 



and the result is, that the coloured fringes are almost 



entirely neutralUeil, the lena becoming perfect for all the 



puri>osos of the optician. 



r\g. si. \Vo shall defer entering into the great value 

 and essential nature of these improvements in 

 the construction of lenses, until we deal with 

 optie.il instruments. It will bo desirable, 

 however, tliat tin.- student should fully master 

 the principles which we have hero explained, so 

 as to avoid the necessity of circumlocution, on 

 our part, when we liave to refer to their 

 applications. 



. 31 represents a section of an achro- 

 matic lens ; wherein a double concave lens of 

 flint glass, a, and a double convex ouo of crown 



glass, I, are united together. 



EPIPOLIC DISPERSION. 



A TBOTLIAR kind of dispersion of light is effected by some 

 substances both in a solid and liquid state ; which, from 

 its connexion with the chemical rays of the spectrum, 

 has become a subject of considerable interest in the 

 of Optics. 



If a solution of sulphate of quinine in dilute sulphuric 

 acid be poured into a glass of water, it will be observed 

 that a peculiar blue tint pervades the liquid, whether it 

 is viewed by reflected or transmitted light. The attention 

 of scientific men was first called to this effect by Sir J. F. 

 W. Herschel ; and it has been since fully investigated by 

 several physicists. 



Similar effects to those produced by the solution of 

 quinine may be observed in an infusion of the bark of 

 the horse chesnut ; the green colouring matter of leaves, 

 obtained by leaving them some considerable time in 

 spirits of wine ; a solution of the colouring matter of 

 some kinds of sea-weed ; glass coloured by the oxide of 

 ur.inium, <tc. We have lately noticed the epipolic 

 appearance whilst pouring semi-congealed oil from a glass 

 botlle, and also when the juices of some kinds of lemon 

 are squeezed into a glass of cold water. The cause of 

 this phenomenon is a peculiar and internal dispersion 

 of the refrangible rays of light. Indeed, Professor 

 Stokes shows that the rcfrangibility of some rays is 

 changed. It is found, that light which has thus been 

 affected, has not the power of transmission through 

 a solution similar to that which has been employed to 

 produce it. Thus, a solution of quinine placed in a glass 

 vessel, and viewed through another vessel containing a 

 similar solution, does not present the blue appearance 

 which may be seen when it is viewed alone. In other 

 words, the passage of the epipolised rays is completely 

 stopped by the second solution. The investigations which 

 have been made on this subject have much assisted in 

 illustrating the nature of the rays of light existing beyond 

 the visible portion of the spectrum, where the chemical 

 power of light is found to bo most active. To this we 

 shall further allude under the head of Photography, 

 when we enter into the philosophy of the various processes 

 involved in that interesting application of the effects of 

 light. 



Having thus pointed out some of the chromatic effects 

 resulting from the interference of rays of light and the 

 undulations of ether, as producing these effects, wo now 

 proceed to investigate a series of singular and interesting 

 phenomena connected with mono-chromatism, and the 

 production of Fraiinhofer's lines or spectral bands. We 

 shall also introduce an account of a very interesting dis- 

 covery made by continental chemists ; from which we 

 shall not fail to observe how close is the connexion be- 

 tween the matter with which we are surrounded, and 

 those forces which are at once latent in it. 



MONO-CHROMATIO LIGHT. 



IN previous pages we have supposed that ordinary 

 white light has been employed for our experiments and 

 investigations. Under certain circumstances, how . 

 a variety of interesting phenomena is presented when 



of a single colour is used. Mono-chromatic light, 

 as its naino indii-ati's. Dimply presents one colour to any 



on hich it falls; and such objects, there.ire, only 

 t that colour in its simple or partially modified 



to the eye viewing them. 



As all the objects generally perceived by the eye are 

 seen by the aid of white light, which (with comparatively 

 trifling oxivpiions) is comjiosed of the colours presented 

 by the spectrum; it may be well that we should ailord 

 some instances winch will illustrate the peculiar pheno- 

 mena arising when single-coloured light is similarly 

 employed. For this purpose the following experiments 

 may be tried. 



EJ-J ' . Put a table-spoonful of common salt 



into a glass bottle containing two OUDOM of strong spirits 

 of wine, and agitate them together until as much as 

 possible of the salt is dissolved. 1'otir some of the . 

 solution on a piece of tow or on a sponge, and inflame it 

 in a room where no other light is present. All objects 

 of a light colour will present a pale yellow tinge ; whilst 

 those of a dark tint, such as deep blue, <tc., will apprar 

 of a gray or black colour. The human face presents a 

 most ghastly appearance when viewed by tliis light, 

 simply because it reflects no colour except that afforded 

 by the burning spirit. 



Experiment 5. Hang a few light-coloured ribbons on 

 a stick, and view them by means of the light produced in 

 the last experiment. If light prints, yellow, and orange 

 are used, each ribbon will present the same colour, 

 owing to the mono-chromatic nature of the light employed. 



Esjteriment 6. After observing for a short time the 

 effect of this colour on the ribbons, introduce the light of 

 a candle. Tliis source of light being nearly equal to white 

 light, will represent the original and natural colours of 

 the ribbons. 



As some of our readers may be desirous of repeating 

 this experiment in public, or as a class illustration, we 

 may mention, that the best mode of so doing is, by 

 burning a large quantity of the spirit on a piece of tow ; 

 and instead of employing ribbons, a very effective mode 

 is that of clothing the upper part of the experim 

 with folds of glazed calico of different colours. Tin; 

 effect on the face and the clothing is thus perceived 

 simultaneously. If the oxy-hydrogen light be then 

 suddenly thrown on the operator, or the light of a 

 powerful g;is name, the original colours will be at once 

 restored. They are, of course, immediately lost on the 

 extinction of the white light, and subsequent employment 

 of the mono-chromatic. This, if well managed, forms an 

 excellent experiment for the lecture-table ; and is a good 

 illustration of the effects produced by either the presence 

 or absence of those rays which are essential for viewing 

 bodies in their ordinary state. 



Other solutions of salts in alcohol produce flames which 

 have characteristic colours. Thus a solution of nitrate of 

 baryta or boracic acid in spirits, produces a flame of a 

 green colour ; and nitrate of strontia gives one of a rich 

 red colour. In the same manner these salts afford rich 

 green or red flames when burned with charcoal ; and, 

 under the names of green and red fire, are largely em- 

 ployed in theatrical representations for numerous scenic 

 effects. By the use of these mixtures the transformation 



* of the pantomimes are always illuminated. We 

 have already referred to the effects produced on the natu- 

 ral colour of bodies by the spectrum at a previous page ;* 

 and that, with the experiment we have just mentioned, 

 afford illustrations of the effect of single-coloured light. 



We have supposed that ordinary artificial light is 

 identical in its physical character with that of the sun ; 

 but such is not actually the case. The nearest approach 

 which we have to solar light is that produced by means of 

 charcoal points, forming the terminals of the voltaic 

 battery. Tliis light (commonly called the electric light) 

 is one of great intensity ; and every coloured body 

 irs, as near as possible, of the same tint as when 

 viewed by means of sunlight. The lime light comes 

 next in order, with respect, to the purity of its rays ; but 

 is much inferior to t'.o fc'.ectric light. A very beautiful 

 Ant., p. 49, ft ley. 



