OF VISION.] 



UNDULATOPvY FORCES. LIGHT. 



strong, will thus be decomposed, and become of a dee 

 yellow tint. Chlorine and hydrogen gases will rapid! 

 unite when exposed to the sun's rays. White paint i 

 often darkened only in those places on which solar ligh 

 has long impinged ; and, similarly, the effects of light art 

 observed on nearly every surface which has been artifici 

 ally coated with the colours usually employed by house 

 painters, their hue being either increased or diminished 

 Some of these effects, however, are due partly to othe 

 agencies, which modify, although they do not prevent 

 that to which we have alluded. White-lead pamt thu 

 always becomes of a dark colour, in the presence o 

 (sulphuretted hydrogen gas, which, with the lead, forms i 

 sulphide of that metal. 



Besides being affected by the purely chemical or actinii 

 rays, many plants seem susceptible of what we ma; 

 almost call a nervous influence, from the presence o 

 absence of light. Some flowers droop and close during, 

 full sunshine, and others close in its absence. Instance 

 of this kind are common in our gardens, and may be 

 observed in the flowers of the evening primrose, the 

 m.irvel of Peru, the marigold, the convolvulus, ant 

 several others. Of all these, however, the sunflower is 

 the most extraordinary for its apparent affection for solar 

 li -;lit ; and hence its name. 



LAR DEFECTS IN RESPECT TO COLOUR. 



VIDUAL.S are occasionally met with who are quite in- 

 capable of distinguishing the difference between colours, 

 who ascribe names different to those applied to any 

 ir by persons unaffected by this peculiarity of vision. 

 < 'in' of the most remarkable instances of this kind was 

 that of Dr. Dalton, the eminent chemist; of whom it in 

 amusingly related, that having on one occasion purchased, 

 as he thought, a iiir of gaiters of a drdh colour (he being 

 a member of the "Society of Friends"), was remon- 

 strated with by a "friend," who found him walking 

 through Manchester with those of a flaming retL 



Some interesting experiments are detailed in Sir J. F. 

 Ilerschel's Treatise on Light, which were tried on a person 

 thus affected. So as to secure "the true corn plemeutary 

 colour to each presented to the eye in succession, polar- 

 ised light, transmitted through various thicknesses of 

 plate* of mica, was employed. By thcsu means, it was 

 found th;it the blue and yellow colours were those which 

 were best distinguished ; whilst all others, including pure 

 red and compounded colours, were either confounded 

 with each other, or named differently to their usual titles. 

 Sir J. F. Uerschel sums up the results of this in thu 

 following manner : "It appears that the eyes of the 

 individual in question are only capable of fully appre- 

 ciating blue and yellow tints, and that these names uni- 

 formly correspond, in his nomenclature, to the more or 

 less refnm.'il'le rays generally ; all which belong to the 

 former indifferently exciting a sense of ' blueness,' and 

 to the latter of 'yellowness.'" In some cases red is 

 confounded with green, green with brown and red, and 

 violet with blue. 



It is difficult to give a reason for this strange affection ; 

 but, j>erhaps, it can be accounted for on the supposition, 

 that the defect lies not in the mechanical construction 

 or chemical composition of the humours of the eye, but 

 rather in some affection of the brain, so far as its optical 

 sense exists. Instance* have occurred in which disease 

 has produced the effect We met with a case, some yean 

 ago, of a friend who lost the power of distinguishing 

 colours, and the sense of smelling, owing to a fall which 

 injured the back of the head. The injury, howi-w-r, 

 produced no other | rm.uii-nt result than those we have 

 named. We regret that, owing to the death of this gen- 

 tleman, we cannot furnish more accurate and extended 

 statements of the facts of his case. Considerable benefit 

 would accrue if individuals thus affected would forward 

 statements of their peculiarities of vision to scientific 

 in-n Sii'-h would not only add to our own stock of 

 facts, but, by the variety of cases, it might happen that 

 Rome ri-ineily would eventually be discovered. This 

 might be done without the publication of the names of 



individuals; their statements being guaranteed by a 

 respectable medical practitioner, as I precaution against 

 incorrect detail or wilful misrepresentation. 



OPTICAL INSTRUMENTS. 



WE now proceed to apply the laws which we have been 

 previously explaining ; and if our readers have succeeded 

 in mastering the statements and doctrines we have been 

 investigating, they will have little difficulty in understand- 

 ing the use of the philosophical instruments we are now 

 about to describe. Although in these we shall find alike 

 the toy of the child and the splendid instrument of dis- 

 covery of the astronomer, still each and all depend, both 

 in their use and construction, on the laws of the reflec- 

 tion and refraction of light. 



Still pursuing our plan of passing from known to un- 

 known matters, we commence with the Camera Obscura, 

 as one of the simplest of optical instruments. We shall 

 then pass on to the Magic Lantern, Phantasmagoria, Dis- 

 solving Views, <tc. ; thence to Microscopes, simple and 

 compound the oxy-hydrogen and solar ; Telescopes, re- 

 fracting and reflecting ; and shall then describe minor 

 arrangements, such as the Kaleidoscope, Debusscope, 

 Kalotrope, the use of concave and convex mirrors, with 

 others which we need not mention here. 



Besides detailing the construction of the instruments, 

 we shall give extended directions for their general ma- 

 nagement, repair, A'c. : affording our readers the results 

 of some experience in the practical use of each, under 

 the belief that many persons who may peruse our pages 

 will l>e desirous of availing themselves of as much infor- 

 mation as we can offer on the subject. 



THE CAMERA. 



of the most simple and easily constructed of all 

 optical instruments, is the camera obscura ; and of this 

 ;here are several kinds. The higher class of instrument, 

 is chiefly employed for photographic purposes ; and as 

 ;reat attention must )>e paid to the character and make 

 >f the lenses and their adjustment ; and as its value also 

 s only to be properly appreciated when employed for 

 ;he purposes we have named, wo reserve its consideration 

 'or that branch of our remarks included under the head 

 >f Photography. 



Fig. M. 



A camera obscura, for ordinary purposes, may be thus 

 made : Into one end of an oblong box, A, say a foot 

 ong, eight inches wide, and six inches deep, fix a double 

 convex lens as at a, in Fig. 34, which gives a sectional 

 view of the instrument. A piece of British plate look- 

 ng-glass must be fixed at an angle of 45, as at E, so as 

 to receive the rays of light passing from the lens a, and 

 it a distance dependent on the focal length of the lens. 

 The top of the box mtujt have a flap fitted into it, and 

 ixed on a hinge as at B C ; and between B and the edge 

 of the box a piece of ground glass or thin paper must 

 est, so as to receive the rays passing from the reflecting 

 nirror E, and which arc thus spread over the glass at F. 

 'lie inside of the box should be blackened, so as to pre- 

 vnt the reflection of its sides from marring the picture, 

 f an arrangement of this kind be placed with the lens a 

 turned towards any object, tke ima^e of the latter will 



