40 W. H. FURLONGE ON MONOCULAR VISION. 



be made, and that to produce the best effect it is necessary to screen 

 the jDaper from too bright a light. 



8ome months since, in attempting to accomplish this adjustment, 

 I chanced to hit upon a singularly happy illumination of the paper, 

 by means of which I was enabled to trace the object under obser- 

 vation with more than ordinary facility. On removing my eye from 

 the instrument, however, I was greatly surprised to find that in 

 shading my drawing paper to produce the best effect, I had actually 

 interposed the margin of the screen between the observing eye and 

 the drawing-paper. I had thus completely cut off all possibility of 

 vision of the reflected image through the neutral tint glass with 

 the observing eye ; yet the apparent projection of the image upon 

 the paper was seen with increased distinctness. Repeated experi- 

 ments only served to confirm the reality of this singular pheno- 

 menon of vision, and to prove that the best mode of using the 

 reflector as a drawing instrument was to employ a piece of glass 

 which had been made perfectly opaque by the application to its 

 under surface of a thick coating of black varnish, such as asphaltum, 

 instead of the transparent neutral tinted glass. 



Now let us consider the rationale of this curious fact. It will 

 be at once perceived that we must entirely modify the views hitherto 

 held as to the principles upon which we are enabled to perceive the 

 image reflected by the neutral tint — and, in fact, by every other 

 form of reflector — upon the paper on which we draw. These 

 principles I take to be, that the pencils of light forming the image 

 in the microscope are reflected into the observing eye, which, look- 

 ing through the transparent reflecting glass, sees the image appa- 

 rently projected upon the drawing-paper beneath — the office of the 

 non-observing eye being to direct the point of the pencil while the 

 object is being traced. But the experiment I have described 

 proves that the observing eye does not see through the reflecting 

 glass at all, and that in reality it is the non-observing eye — that is 

 the eye which is not looking into the instrument — that perceives 

 the image, and is called upon simultaneously to direct the pencil. 

 This at once explains the difficulty we all feel, more or less, in 

 keeping the point of the pencil constantly in view, and why it is 

 that in tracing an object we are so continually and provokingly 

 losing sight either of the pencil or of the object. 



I do not propose here to enter upon the somewhat abstruse 

 enquiry, how the mental impression conveyed to the brain by one 



