in contact with the tube. The dissimilar objects producing unlike im- 

 ages upon the retinae, the sensations blend, and a hole will appear to be 

 cut through the palm of the hand, through which the tube passes. That 

 part of the tube between the eye and hand will appear to be transparent, 

 as though the hand were seen through it. 



This experiment is very old, but seems not to have found its way into 

 scientific literature. 



2. Replace the hand by a sheet of unruled paper, upon which a drop of 

 ink has been placed. By proper management, the ink-blot may be made 

 to appear within the tube, by so placing the paper that the hole, which is 

 apparently cut through it, coincides with the blot. Ordinarily the blot 

 will then appear opaque, the paper immediately around it and apparently 

 within the tube being invisible. The blot appears as it were suspended in 

 space. By concentrating the attention strongly on objects seen through 

 the tube, especially if they are strongly illuminated, the blot becomes more 

 hazy, transparent, and may even be made to disappear altogether. The 

 mental effort necessary to do this cannot be maintained more than a few 

 seconds, and the spot will reappear. If the effort to cause the spot thus 

 to disappear be kept up, the attention being strained to its highest pitch, 

 the blot will disappear and reappear at regular intervals of a few seconds, 

 the absolute time depending upon the illumination. It appears as though 

 the organs exerted become fatigued, and, relaxing for a few moments, re- 

 freshment sets in, which again renders possible the exertion necessary in 

 causing the blot to disappear. It is possible that these experiments may 

 be so made as to throw some light upon the conditions necessary in fixing 

 the attention. Interesting experiments may also be made by substituting 

 a fragment of a plane mirror for the sheet of paper. Looking through 

 a rather large tube at a distant object with the right eye, the reflected 

 image of the left eye will appear staring up the tube, the adjoining parts 

 of the head being invisible. 



3. Substituting for the ink-blot a small hole cut through the paper, and 

 the small hole can also be made to appear within the tube, distinguishing 

 itself by its different illumination, the surrounding paper being invisible, 

 unless attention be directed too strongly to the paper in which the hole is 

 cut. The relative illumination of the small hole and the space immediately 

 around it depends upon the relative illumination of objects upon which the 

 tube is directed, and that of the sheet of paper exposed to the other eye. 



4. Keeping the same arrangement, place at a distance of one foot from 

 the end of the tube a sheet of paper, so that objects beyond it are still visi- 

 ble; arrange matters so that it is visible to the eye looking through the 

 tube, but not to the other, directed at the small hole in the paper sheet. 

 This second sheet will now appear to be traversed by a hole the same in 

 size as that cut through sheet No. 1. 



Cutting a small hole in sheet No. 2, matters are easily arranged so that 

 it appears within the hole which was before seen within the tube. These 

 experiments may be utilized in showing the simultaneous accommodation 

 of the two eyes. 



