78 



(111 



LIMIT. 



[PH08PUOKE6CKNCE, ETC. 



dldaront speed or in contrary direction. The result is, 

 that the eye oerooiroi the liu'us on Uio duo at variable 



i , - 



intervals ; and the intersection of the perforations on the 

 screen, has the effect of producing to the eye the ap- 

 pearance of every form of geometrical curves, conic 

 sections, itc. The results are exceedingly pleasing, espe- 

 cially when viewed by means of a powerful light. The 

 arrangement is not susceptible of illustration by means of 

 an engraving; and the instrument has not commanded 

 that attention which its simplicity and beauty entitle 

 it to. 



With this we must conclude our description of optical 

 instruments, and pass on to another and important 

 branch of the science of light, in which we shall have to 

 deal with double refraction an Wo have 



preferred to describe the application <>f li.Jit, so far as 

 we have gone, in this portion of our work, because the 

 instruments we have named are constructed and used 

 in obedience to the principles and doctrines already 

 defined, and have but a remote connection with the 

 phenomena developed by double refraction. It may, 

 p. rhaps, b<> as well if we give a kind of | .<um^ 



of some of the chief points of the affections of light to 

 which wo have hitherto directed the attention of our 

 readers, so that, at all events, the student may retain 

 a knowledge of the subject Bullicient to guide him to 

 those which we have yet to bring to his notice, each 

 of which consists of special applications of previously 

 enunciated truths, in an extended and practical I 

 It so happens that we shall have to introduce i 

 matters, in connection with chemical science, with irhiob 

 some of our readers may be unacquainted. We Khali, 

 however, do so in an elementary manner, and, there- 

 fore, will not have to call for anything beyond the use 

 of an ordinary intelligence in making our remarks under- 

 rtood 



At the comnii-iic..incnt of this section, we first remarked 

 on light as a latent force, and found that, although its 

 chief source U the sun, still, by proper means, it could l>e 

 1 from a latent state in bodi.-s by certain processes ; 

 it is, i.-i Ibis, and in oth. i 



to heat and el.-.-tiirity. \\,. thc-n proceeded to show 

 that white light is compounded of . , s, which 



may easily be obtained by aid of the prism. We o).-,-, \, ,1 

 that each of these rays, or waves of light, varied from 

 the rest, both with respect to the. length of its undu- 

 lations, their number within a given time, and their 

 refrnngibility. 



.11, ami its laws, were next revi 

 of plane, spherical, parabolic, and ellij 

 reflectors were i -1 this branch of the- s. 



that of Catoptrics. We next 'hat a i.iy 



lit, pausing into or out of media of dilli rent den- 

 sities, undergo. n, or an alteration of its pro- 

 gress from a straight line, if tii incident < 

 refracting lx>dy at any perpendicular 

 direction. As an expansion and application of this law, 



we next entered into an examination of \arious forms 



uses, and the direction which a ray of light taken 



through them. That p, i lenticular arrange- 



1 in i nan e\. toge- 



.. peculiarities of vision to which some 



persons are subject. 



The phenomena of colour, we stated, wen- produced 

 by the absorption ,.f all the rays of h-Jit except ih.it 

 which was reflected by tin- Mirface of fhe lx.dy to tin- 

 eye; and we noticed that, whether Mich effects of colour 

 were produced by reflection from a body, or by trans- 

 mission through its substance, still 



was due to the retention that 



perceived by us. The colours of thin plates of air as- 

 sisted us to measure the length and frequency of the 

 undulations of each ray presented by the prismatic 

 spectrum. We tin i n of com- 



plementary colours, to which wo shall have constantly 

 :.-r when dialing with polarisation. 



Diffraction, dispersion, and achromatism then came 

 under review, together with a peculiar form of disper- 

 sion, called epipolic. 



Under the head of mono-chromatic light, we were 

 enabled to discuss the effect of a single coloured light 

 on surrounding objects, whereby we noticed tli:r 

 colour of a body depended on that of the light incident 

 on its surface. 



An interesting discovery of Fraiinhofer, that there 

 are certain marks or bands in the sp.-ctrum, and an 

 extension of these facts in the recent discoveries of 

 Messrs. Buuson and Kirchkoff, next engaged our atten- 

 tion. Wo shall add a supplemental chapter on this 

 subject, as the best mode of giving a full account of ii- 

 interesting facts which have been daily discovered, since 

 those named in the remarks wo made went to press. 



We then described some of the effects of light on 

 organised bodies ; showing how animal and vegetable life 

 are variously and powerfully influenced by that agent. 



Having noticed some special ocular defects from w hi. h 

 some persons suffer in respect to distinguishing colours, 

 we subsequently explained, as an application of our 

 previous remarks, the construction of the foil. 



;1 instruments: namely, the camera obscura ami 

 lucida; the multiplying glass; the magic lantern, 

 solvhi s, and interesting objects 



to which they can be applied; telescopes, both rei 

 ing and reflecting; concave and convex mirrors; 

 tocles; magnifying and burning-glasses; the kaleido- 

 scope; the Dobusscope; the thaumutrope; the plian- 

 tasmascope; an.: .alotrope. 



We may hero make a few remarks on the peculiar 

 phenomena which we shall more particularly deal with 

 in our section on chemistry: we refer to plio.-j,' 

 and fluorescence. Phosphorus and fluor-spar, with many 

 other bodies, have the power of spontaneously cnn 

 light; and in the case of phosphorus, this is owing to a 

 slow combustion, or its union with tl lined 



in the atmosphere. This effect, however, may be ob- 

 served in many animal substances ; hence the [ominous 

 appearance of . and the pliosp]:. of the 



uhich proceeds from extremely minute, ov:-. : 

 swimming on its surface. 



Fb. has obtained its name from the spar we 



have named ; I that sub.-: 



The two terms are frequently applied to ;. 

 sinii! rred to, but arising from 



of th. LTge. \\ e can fully imdi rstand that 



the student may tind some difficulty, whilst .-..i, .-. 



the phenomena of which wo are taking notice, in re- 



;uction of . 



with the n marks wo have previously made on tho 



forces. Indeed, it would seem, at 



first sight, that light, in such c . but 



n mind, that el .illy a. 



form of motion, thai which one 



-lit may 1 Bother; and just as the sun 



and the earth have a mutual attraction lion, 



and BO tend to approximate the chemical attraction 



