NATURAL PHILOSOPHY. 149 



phantom to the object, as the distance between the eyes is to the dis- 

 tance between the identical pictures converged together. It follows, 

 that, any three of the quantities being given, the fourth can be calculat- 

 ed either by proportion or by equation. By this means I calculated 

 the diameter of the floor-cloth panels, the result being within one 

 fourth of an inch of the actual measurement. I mention this merely 

 as an illustration of the subject for the base line ; the distance between 

 the eyes is too short for practical use." 



This instrument shows that we do not see an object in itself, but the 

 mind contemplates an image on the retina, and always associates an 

 object of such a figure, attitude, distance, and color, as will produce 

 that image by rectilinear pencils of light. If this image on the ret- 

 ina can be produced without the object, as in the phantascope, then 

 there is a perfect optical illusion, and an object is seen where it is not 

 Nay, more, the mind does not contemplate a mere luminous image, 

 but that image produces an unknown physiological impression on 

 the brain. It follows, that if the nerves can, by disease or by the 

 force of imagination, take on this action, a palpable impression is 

 made without either object or picture. As this would be most likely 

 to occur when actual objects are excluded, as in the night, we have 

 an explanation of the scenery of dreams, and the occasional " appari- 

 tions" to waking persons. 



USE OF COLORED GLASSES TO ASSIST THE VIEW IX FOGS. 



M. Luvrxi, of Turin, in a letter to the editor of Vlnstitvt^ at 

 Paris, makes the following curious observation, which, if confirmed, 

 may prove to be of great importance : " When there is a fog be- 

 tween two corresponding stations, so that the one station can with 

 difficulty be seen from the other, if the observer passes a colored glass 

 between his eye and the eye-piece of his telescope, the effect of the 

 fog is very sensibly diminished, so that frequently the signals from 

 the other station can be very plainly perceived, when without the 

 colored glass even the station itself is invisible. The different colors 

 do not all produce this effect in the same degree, the red seeming to 

 be the best. Those who have good sight prefer the dark red, while 

 those who are short-sighted like the light red better. The explana- 

 tion of this effect seems to depend upon the fact, that the white color 

 of the fog strikes too powerfully upon the organ of sight, especially 

 if the glass have a somewhat large field. But by the insertion of the 

 colored glass, the intensity of the light is much diminished by the 

 interception of a part of the rays, and the observer's eye is less wea- 

 ried, and consequently distinguishes better the outlines of the object 

 observed." 



OX AX UXNOTICED KIND OF ABNORMAL VISION. 



PROFESSOR C. DEWEY communicates to Silliman's Journal, for 

 November, the following notice of a new kind of abnormal vision. 

 There are two well-known kinds of abnormal vision in eyes not dis- 



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