1868.] q6 [Jeffries. 



tion that has been known on the Hawaiian Islands, are as yet very 

 incomplete, owinjr to the constant i'car and anxiety of the people, 

 and the absence of any geologist or scientific man. 



Dr. B. Joy Jeffries stated that he had followed out the 

 experiment in regard to the projection of after-j)ii'tures, 

 spoken of by the President at a previous meeting. 



The projection, for instance, of a circle, will be a right angled 

 cone, its apex at the nodal point and its base in space. All surfaces 

 cutting this jjrojected cone at an angle to the perpendicular give 

 conic sections, thus explaining the after-picture of a circle appear- 

 ing as an ellipse, of a square as a lozenge, etc. Dr. Jeffries, in 

 continuing the experiments, had found them very interesting, as 

 showing the mentality of the eye; for, notwithstanding he projected 

 against a surface at an angle to the line of vision, yet by mental 

 effort he could recall the circle as if against a surface at right angles 

 to the hne of vision, or again let the ellipse be formed. A certain de- 

 gree of excited sensibility of the retina, so favorable to all after-pic- 

 tures, assisted the experiments; but whether the circle or ellipse was 

 seen, seemed to depend upon intentness of mental action, somewhat 

 as we recall or suppress the picture in either eye, when using the 

 monocular microscope or ophthalmoscope. Dr. Jeffries said that his 

 colleague. Dr. Hay, in testing the experiment, saw the circle through 

 or beyond the surface, cutting the line of vision at an angle. The 

 President stated that this was also the case with himself Dr. Jeffries, 

 however, saw the circle touching with its edge the inclined sur- 

 face, therefore in front of it. He exhibited the disks and circles used 

 in experimenting, white, black, and some of the primitive colors, in 

 order to have a complementary colored circle or ellipse, as better con- 

 trasted with the first, in the after-picture. He made some remarks 

 in regard to after-pictures, stating that he could not find mention of 

 the above experiments in any optical treatises or physiologies. He 

 explained how after-pictures are produced and suppressed, and bv 

 means of diagrams, illustrated the theory of projection. Such ex- 

 periments, however simply curiout they might seem, often lead to 

 important and valuable discoveries. These after-pictures had quite 

 recently been employed by Dr. Giraud-Teulon, to obtain necessary 

 data in regard to the contraction of the field of vision in a case where 

 the latter was of great diagnostic value. 



