Dioptrie System. —Another method of bending the diverging rays proceeding front a lauip into such directions as shall be useful to the mariner is that of refraction. If a flame be placed in the focus of a lens of the proper form, the diverging rays will be bent parallel to each other, so as to form a single solid beam of light. M. Augustin Fresnel was the first to propose and to introduce lenticular action into light-house illu mination, by the adoption of the annular or built lens, which had been suggested as a burning instrument by Buffon and Condorcet. He also, in conjunction with Arago and Mathieu, used a large lamp having four cencentric wicks. In order to produce a revolving light on the lenticular or choptrie system, a different arrangement was adopted from that which we have described for the catoptric system. The large lamp was now made a fixture, and four or more annular lenses were fitted together, so as to form a frame of glass which surrounded the lamp. When this frame is made to revolve round .the lamp, the mariner gets the full effect of the lens whenever its axis is pointed toward him, and this full light fades gradually into darkness as the axis of the lens passes from him. In order to operate upon those rays of light which passed above the lens, a system of double optical agents was employed by Fresnel. These consisted of a pyramid of lenses with mirrors placed above at the proper angle for rendering the rays passing upward parallel to those which came from the annular lens. But Fresnel did not stop here, for, in order to make the lenticular system suitable for fixed as well as revolving lights, he designed a new optical agent, to which the name of cylindric refractor has been given. This consisted of cylindrical lenses, which were the solids that would be gen erated were the middle vertical profile of an annular lens made to circulate round a vertical axis. The action of this instrument is obviously, while allowing the rays to spread naturally in the horizontal plane, to suffer refraction in the vertical plane. The effect of this instrument is, therefore, to show a light of equal intensity constantly all round the horizon, and thus to form a better and more equal light than that which was for merly produced for fixed lights by parabolic reflection. It is obvious, however, from our description that the diverging rays which were not intercepted by this c3rlindrie hoop, or those which would have passed upward and been uselessly expended in illuminating the clouds, or downward in uselessly illuminating the light-room floor, were lost to the mariner; and in order to render these effective Presnel ultimately adopted the use of what has been called the internal or total reflection of glass; and here it is necessary to explain that one of the great advantages of the action by glass over reflection by metal is the smaller quantity of light that it absorbs. It has been ascertained that there is a
gain of nearly one-fourth (.249) by employing glass prisms instead of metallic reflectors for light-house illumination. There were, therefore, introduced above and below the cylin dric refraeng hoop which we have described, separate glass prisms of triangular sec tion, the first surface of each of which refracted to a certain extent any ray of light that fell upon it, while the second surface was placed at such an angle as to reflect, by total reflection, the ray which had before been refracted 1)3- the first surface; and the last or outer surface produced another refraction, which made the rays finally pass out parallel with those refracted by the central cylindric hoop. The light falling above the cylin dric hoop was thus by refractions and reflections bent downward, and that falling below was bent upward, so as to be made horizontal and parallel with that proceeding from the refracting hoop. Fig. 1 represents in vertical section this; which is the most perfect of Fresnel's inventions in light-house illumination, especially when made in pieces of the rhomboidal form, and used in connection with the diagonal framing introduced by Mr. Alan Stevenson. In the fig., p shows the refracting and totally reflecting prisms; and It the cylindric refractor.
From what has been stated, it will be readily seen that, in so far as regards fixed lights, which are required to illuminate constantly the whole of the horizon with equal intensity, the dioptric light of Fresnel with Mr. Alan Stevenson's improvements is a. perfect instrument. But the case is different as regards ::::: lc.
revolving lights, or those where the whole rays have to. _ .'14,110111,..S.4,,,. be concentrated into one or more beams of parallel rays.
- Stiimorne, .%,,,, lignIIIIIIIIhr To revert to the parabolic reflector, it must be obvious ..„,,,kuwwilf,,,...,., _ — that all rays which escape past the lips of the reflector, , _ •,,za•r,:,;,Zijik livogrprit."...2, —: never reach the eye of the mariner, while, if we return #1 1,, to the clioptric revolving light of Fresnel, we find that _A .._ 1,4,„ T„, ,._______. those rays which escape past the lens are acted on by.
0itiM - two agents, both of which cause loss of light by absorp i41 \ i','" i tion. The loss occasioned by the inclined mirrors, and in passing through the pyramidal inclined lenses, • ‘, ... . / .;./ j, :ea i was estimated by Fresnel himself at of the