214 ON ACCIDENTAL OE SUBJECTIVE COLORS. 



Fhotometer of Mr. Wkeatstone. — Another instrument of this distinguished 

 English savant, founded on the same principle, and called a 2^^ot(jmeter, produces 

 still more singular appearances. It consists in causing to revolve eccentrically 

 a disk of cork, having attached to it a greater or less number of little balls simi- 

 lar to those just spoken of, and arranged under the form of regular or irregular 

 polygons. Each of these balls, illuminated by a ray, gives rise in its rotation 

 to a curve, and the different curves, by their involutions, describe the most curious 

 figures. 



Different chronoscopes. — Means of afpireciating the instantaneousncss or dura- 

 tion of various phenomena. — Mr. Wheatstone has also devised some very curious 

 processes, founded in like manner on the persistence of impressions, which have 

 enabled him to prove the instantaneousncss of certain luminous phenomena, such 

 as the electric spark, or to estimate their duration, however short. One of them 

 consists in observing the phenomenon by reflection in a mirror, to which a very 

 rapid movement is given of such a nature that supposing the luminous object to 

 be persistent, its image seems to describe a great circle. Now, if the phenomenon 

 be instantaneous, the image can be seen only in a single point of this circle, and 

 will not appear changed in form ; if, on' the contrary, the phenomenon has an ap- 

 preciable duration, the image will be elongated in such a manner as to form an 

 arc of a length proportioned to the duration, so that from the magnitude of the 

 perceived arc the duration of the luminous phenomenon may be calculated. An 

 electric spark observed in this manner does not appear at all elongated, whence 

 it must be regarded as instantaneous. Mr. Wheatstone has, in this way, rendered 

 perceptible the intermission of certain flames or luminous jets which, to the rlaked 

 eye, seem perfectly continuous, such as a current of electric sparks received at a 

 very small distance from a conductor. The light being thus, in an extremely 

 small interval of time, stretched along a greatly extended line, the slightest dis- 

 continuity becomes visible, and it is for this reason that the current of sparks 

 presents under such circumstances the aspect of a series of luminous points sepa- 

 rated by dark spaces. 



Again, Mr. Wheatstone has demonstrated, in the following manner, the instant- 

 aneousncss of the electric spark : A disk of white pasteboard, on which is drawn 

 in black any figure whatever, is made to turn with great rapidity in its own 

 plane around a fixed axis. From the movement of rotation it becomes impossi- 

 ble any longer to distinguish the figure traced on the circle, which now presents 

 only a sei'ies of concentric bands of different tints. This is an evident result o£ 

 the persistence of impressions. Now let the circle be put in motion in a cham- 

 ber perfectly darkened, and be then suddenly illuminated by an electric spark, 

 immediately the figure will be very distinctly perceived, as if it were in a state 

 of complete immobility, no matter what velocity may be given to the circle. But 

 this effect could not take place if the spark had a sensible duration, for then the 

 figure would be seen in several successive positions, and thence a confusion would 

 result, so much the greater as the duration of the spark was more prolonged. Mr. 

 Wheatstone has further availed himself of this process. to destroy, in certain cases, 

 the false appearances which the persistence of impressions gives to objects in rapid 

 motion. These objects being illuminated by this means, but for an exceedingly 

 short time, they exhibit themselves only in a single position, and 'the impression 

 which they produce having a sensible duration, we are enabled to judge of their 

 true form. It is thus that Mr. Wheatstone has succeeded in seeing very dis- 

 tinctly with the microscope the movable wheels of the infusory animal called 

 the rotfer. . The foregoing processes will be found described at length in our 

 treatise on electric telegraphy, as well as the uses to which MM. Arago, Wheat- 

 stone, and Dove have applied the same principles, in order to make manifest the 

 duration of flashes of light, the discontinuity of their illumination, the velocity of 

 electricity, the unequal velocity of light in different refracting mediums, &c. 



