Page:The American Cyclopædia (1879) Volume XIII.djvu/490

 474 PHOTOMETRY PHRENOLOGY will appear darker than the surrounding space ; from the other side, lighter. When the light falling on each side of the paper is equal, the spot and the surrounding surface will present the same shade, and the squares of the dis- tances of the sources of light will respective- ly indicate their intensity. A modification of this instrument by Dr. Letheby is in common use in gas works. Masson's electro-photometer has the advantage of comparing lights of differ- ent colors. A circular disk, divided into equal white and black sectors, makes by clockwork from 250 to 300 revolutions a second. When illuminated by a constant source of light, the disk has a uniform gray tint, because of the blending produced by the duration of the vis- ual impression ; but when lighted instantane- ously by the electric spark, the black and white sectors appear distinct and as if fixed. If now the constant light be continued, the intensities of two electric lights, of the same or of differ- ent colors, may be compared by removing one and then the other from the disk, until the latter appears uniformly gray. Conversely, two constant lights may be compared by car- rying each toward the disk till it is uniformly gray, the illumination being primarily made by one source of electric sparks. The square of the distances in either case will be proportion- al to the intensities of the lights which are the subjects of examination. The general conclu- sions obtained from photometric observations are that flame is perfectly transparent to light ; so that the luminous effect of a series of flames placed side by side is the same whether the light is received in a direction perpendicular or parallel to the line in which they are placed. In like manner a flat gas flame, such as that of a bat's-wing burner, casts the same amount of light in all directions. It has been found that when two lights are compared, of such relative intensity as that one should cast a shadow on a screen 64 times as strong as that of the other, the lesser light will in reality cast no shadow at all. Therefore it is concluded that light emanating from any source becomes impercep- tible in the presence of light having 64 times its intensity. Hence, whenever the sun's light in the morning becomes 64 times as strong as that of a star, the latter will disappear ; and in the evening, when it becomes less than 64 times that of a star, the latter begins to be visible. When two flames overlap each other, the intensity of the combined flame is great- er than the sum of the two separate flames. This effect, first observed by Franklin, appears to be due to the increased temperature of the part where the flames overlap. Arago and Fres- nel constructed lamps for lighthouses on this principle, in which several concentric wicks were arranged near to each other. CHEMI- CAL PHOTOMETERS. At the end of the article on LIGHT mention is made of investigations in regard to the measurement of the chemi- cal action of light by Dr. John W. Draper, in which he obtained his results by determin- ing the amount of hydrochloric acid formed in a given time from the union of its gaseous elements under the influence of light. Bun- sen and Roscoe have since extended these ex- periments, and have produced an instrument on the same principle which is capable of the most delicate measurements. It consists of three parts: 1, an apparatus for generating the two gases by electrolysis of hydrochloric acid ; 2, one for exposing the gases to light ; and 3, one for measuring the amount of hydrochloric acid generated by recombinations of the hydrogen and chlorine. The gas-gene- rating apparatus consists of a battery of three or four cells connected with two carbon elec- trodes immersed in a vessel containing hydro- chloric acid. The gases are passed through waste bulbs containing water into an insola- tion vessel, which has its lower part blackened, receiving the light upon its upper surface, by which the gases are made to combine. Be- yond the insolation vessel is a measuring tube, beyond this a vessel of water, and after this one containing slaked lime and charcoal, to absorb chlorine. The gases are first passed through the whole apparatus until the water is saturated, when the insolation vessel is ex- posed to the light and the connection between it and the generating apparatus closed. Com- bination of the hydrogen and chlorine gases then takes place with diminution of volume, which causes the water to return through the measuring tube. The rate of movement of the water shows the diminution of volume, and consequently the amount of chemical union. Their unit of light was obtained by employ- ing a flame of pure carbonic oxide gas burning from a platinum jet of a certain diameter and issuing at a certain rate. The apparatus is useful in measuring the chemical action of dif- fuse daylight, of direct sunlight, of flames from different sources, and of the different parts of the solar spectrum. A chemical photometer for registering meteorological changes was also devised by Bunsen and Roscoe, and called a pendulum photometer, by which sensitive paper was exposed to the action of light for a definite time by means of the oscillation of a pendulum. This instrument was used to mea- sure the varying intensity of the sun's light under different positions of the sun, and of conditions of the atmosphere. Descriptions of these instruments and of the experiments made with them may be found in the " Philo- sophical Transactions " of the royal society of London for 1857, '59, '62, and '63. PHRAORTES. See MEDIA. PHRENOLOGY (Gr. Qpfo, mind, and Myoq, discourse), a system of philosophy of the hu- man mind, founded on the physiology of the brain. As a system, it had its origin in the ideas and researches of Franz Joseph Gall, a German physician. First announced by him in 1796, it began to attract attention in Eng- land about the year 1815. It was first dis- tinctly introduced into the United States by