The sheet of paraffined paper, technically i known as the Bunsen disc, is usually mounted in a wooden box with blackened walls (the pho tometer carriage), which slides or rolls along the track between the sources of light. Two small mirrors (M and M', Fig. 5), mounted obliquely within the box, enable the operator to observe simultaneously the two faces of the paper (0.0.).
In practice the paper is frequently used with an unoiled central disc, the remainder of the surface being rendered translucent by treatment with oil or paraffin. To avoid the use of oil or paraffin which gives a surface which soon i becomes soiled from dust in the air, two similar paper screens are sometimes employed. Identi cal portions, in form either a disc or a star, are cut out from the centre of each. A sheet of tissue paper placed between the two then affords a translucent region which takes the place of the oiled paper of Bunsen's original de vice.
The various forms of photometer already described and all others which depend upon the power of the eye to detect slight inequalities of illumination are essentially of equal sensitive ness. They are all limited by the sensitiveness of the eye and approach the maximum degree of delicacy as we fulfil more and more nearly the conditions under which the eye can he used to the best advantage. All forms of the Ritchie and Bunsen photometers, of each of which many modifications have been devised, are, however, subject to a peculiar source of error. It is found that when two sources of light, the rela tive intensities of which are known, are com pared by means of such photometers, observers will persistently set the instrument to a false position. The majority of observers find ap parent equality at a position toward the left hand source. 'The errors which are constant for a given observer, however great his experience, range from 1 per cent to 10 per cent. They appear to be analogous to the errors which oc cur when one endeavors to bisect a straight line by the eye without the use of instruments. The existence of this personal error has led to the substitution, whenever extreme precision is de sired, of a form of photometer in which only one eye is used and in which the conditions of maximum delicacy are more completely met than in any of the earlier types of instrument.
The photometer in question was invented by Lummer and Brodhun. In this photometer as in the instruments of the Ritchie and Bun sen types, the two sources of light to be com pared are mounted at the end of a graduated bar or track. Upon this runs the photometer carriage, the essential features of which are as follows : 1. An opaque screen (0. 0, Fig. 6) upon the whitened faces of which the light from the two sources shines; each face of the screen being illuminated exclusively from one of the sources of light.
2. The device by which both faces of the screen can be observed simultaneously without the use of both eyes. This consists of two precisely similar mirrors M, AC, mounted facing each other within a dark box not shown in the diagram. The diffusely reflected rays from the screen reach these mirrors at an angle of 45' and are reflected as shown in the figure. Where the reflected rays cross each other at an angle of 90 degrees, is placed an ingenious arrangement technically known as the Lummer-Brodhun body, (BDEF) which consists of two right angled reflection prisms placed with oblique faces together so as to form a cube. The face of one of them is, however, cut away so that contact occurs only over a small circular space in the middle of the face. Light from the source S (Fig. 6) diffusely reflected from the left-hand face of the screen and received upon the mirror M, enters the body through the face AB. Those portions of the beam which reach the central face of contact pass through uninterrupted, while all other portions are intercepted by total reflection within the prism. Light from s, dif fusely reflected from the right-hand face of the screen, is reflected by the mirror M' to the face DE of the body. Those portions which fall up on the central area of contact pass through, but the surrounding portions are totally reflected and leave the body in paths parallel to and sur rounding the path of the transmitted beam which has come through the face of contact from the mirror M. Observations are made by means of a small telescope T, the field of view of which consists of a disc of light coming from S, surrounded by a ring of light from s. If the illumination of the side of the screen facing s be brighter than that upon the opposite face of the screen, which receives its light from S, one sees a bright disc of light surrounded by a dark ring. When the position of the photometer car riage upon which the screen, mirrors, body and telescope are mounted is moved to a position such that the two sides of the screen are equally illuminated, disc and ring are equally bright. The distances from the two sources are then read upon the scale of the photometer bar and the relative intensities of the sources are com puted. The accuracy of this form of photom eter depends upon the following conditions: (1) The opaque screen must stand in the line joining the two sources of light. (2) Its two faces must be identical in character both as re gards color and power or diffuse reflection. (3) The mirrors M and M' must be identical as regards reflecting power and must be sym metrically placed. (4) The Lummer-Brodhun body must be optically perfect so that transmis sion through the faces of contact will be com plete.