NATURAL PHILOSOPHY. 147 



hinder disk is so situated as to show the colors through the apertures, 

 the spectators look at a fixed point on the front disk till the eyes are a 

 little weary, when the operator suddenly removes the colors, and ren- 

 ders the whole surface white ; but instead of appearing white, the 

 three apertures are seemingly occupied by a beautifully dilute tint of 

 the complementary color. 



SURVEYING INSTRUMENT. 



A PATENT has been granted for an instrument for measuring dis- 

 tances in reconnoissances. Upon a tripod stand is mounted a hori- 

 zontal axis, from which extends at right angles a radial arm, bearing 

 at its extremity a telescope having its line of collimation parallel with 

 the axis. The telescope is counterbalanced by weights on the oppo- 

 site side of the axis. By making the axis revolve with the telescope 

 and counterpoises, the telescope is with great facility and precision 

 made to assume in succession two parallel positions, at the distance 

 of twice the length of the radial arm on which it is mounted. On ob- 

 serving an object through the telescope in these two positions, the 

 parallax will be apparent at a great distance, and its angular magni- 

 tude is measured by a micrometer attached to the telescope. The 

 parallactic angle thus obtained gives the distance of the object, either 

 by calculation or by reference to a table constructed for the instru- 

 ment. It is said that this instrument may be made to measure a dis- 

 tance of forty or fifty miles or more. Patent Office Report, 1849. 



PRODUCTION OF LUMINOUS IMPRESSIONS ON THE EYE. 



IN a communication made to the Royal Society of Edinburgh, Mr. 

 William Swan gives the following as the result of an investigation 

 undertaken by him to ascertain the relation between the apparent 

 brightness of a light and the time during which it acts on the eye. 

 1. When the eye receives, from a light of common intensity, a suc- 

 cession of flashes of equal duration, which succeed each other so rap- 

 idly as to produce a uniform impression, this impression will also 

 have a constant intensity, provided the number of flashes in a given 

 time varies inversely with the duration of each flash. 2. The bright- 

 ness of the impression produced by flashes of light of a given inten- 

 sity, which succeed each other as above, is proportional to the number 

 of flashes in a given time. 3. When light of a given intensity acts 

 on the eye for a short space of time, the brightness of the luminous 

 impression on the retina is exactly proportional to the time during 

 which the light continues to act. This law has been proved to be 

 true for impressions lasting from 1^32 to 25 of a second. The intensity 

 of the impression produced by light which acts on the eye for .01 of a 

 second, is almost exactly one tenth of the brightness of the light when 

 seen by uninterrupted vision ; and it is also ascertained that light re- 

 quires about the tenth part of a second to produce its full effect on the 

 eye. 4. Lights of different intensity act on the eye with equal rapid- 

 ity. 5. Rays of different refrangibility act on the eye with equal 



