July 1, 1SGS.] 



HARDWICKE'S SCIENCE-GOSSIP. 



14£ 



SENSORIAL VISION. 



HERE is a 

 chapter in Sir 

 John He r- 

 schel's lately- 

 p u I) 1 i s h e d 

 volume of 

 " Lectures on 

 Scientific Sub- 

 jects," which treats of cer- 

 tain peculiar forms of ocular 

 spectra, under the above title. 

 The spectra here alluded to 

 — those which present them- 

 selves to us independently of 

 the will in darkness or when 

 the eyes are closed — are 

 familiar to us all ; but it ap- 

 pears to me that the subject 

 has certain bearings which 

 have been hitherto over- 

 looked, and which merit a passing notice. 



In the first place, I must beg permission to quote 

 Sir John's own words respecting the most frequently 

 occurring forms— those possessing perfect geome- 

 trical regularity : — 



"I find them," he says, "to be formed in darkness, 

 and if the darkness be complete, equally with open 

 or closed eyes. 



" The forms are not modified by slight pressure 

 on the retina, but their degree of visibility is much 

 and capriciously varied by that cause. They are 

 very frequent ; in the majority of instances, the 

 pattern presented is that of lattice-work, the longer 

 axis of the rhombs being vertical. Sometimes, how- 

 ever, the larger axes are horizontal. The lines are 

 sometimes dark on a light ground, and sometimes 

 the reverse. Occasionally at their intersection ap- 

 pears a small close and apparently complete piece 

 of pattern-work, but always too indistinct to be 

 clearly made out. 



" Occasionally the latticed pattern is replaced by 



a rectangular one, and within the rectangles occurs 



in some cases a filling-up of a smaller lattice-pattern 



or of a lozenge of filigree-work, of which it is im- 



No. 43. 



possible to seize the precise form, but which is 

 evidently the same in all the rectangles. 



" Occasionally too, but much more rarely, complex 

 and coloured patterns like those of a carpet appear, 

 but;««f of any carpet distinctly remembered or lately 

 seen ; and in two or three instances in which this 

 has been the case the pattern has not remained con- 

 stant, but has kept changing from instant to instant, 

 hardly giving time to apprehend its symmetry and 

 regularity, befoi-e being replaced by another ; that 

 other, however, not being a sudden transition to 

 something totally different, but rather a variation on 

 the former." 



Thus far I have spoken of rectilinear forms ; with 

 myself, however, curvilinear forms more frequently 

 present themselves. These so closely resemble the 

 spectra which Sir John describes as having presented 

 themselves to him when under anaesthetic influence, 

 that I again quote his words : — 



"The indication," he says, "by which I knew it 

 (the chloroform) had taken effect consisted of a kind 

 of dazzles, immediately followed by the appearance 

 of a very beautiful and perfectly symmetrical Turk's- 

 cap pattern formed by the intersection of a great 

 many circles outside and tangent to a central one. 

 It lasted long enough for me to contemplate it so as 

 to seize the full impression of its perfect regularity, 

 and to be aware of its consisting of exceedingly deli- 

 cate lines, which seemed, however, to be not single but 

 close assemblages of coloured lines not unlike the 

 delicate fringes formed along the shadows of objects 

 by minute pencils of light. The whole exhibition 

 lasted, so far as I could judge, hardly more than a 

 few seconds." 



On the administration of chloroform a second 

 time, after an interval of many months, " the Turk's- 

 cap pattern again presented itself on the first impres- 

 sion, which I watched with some curiosity; but it was 

 not quite as complete, nor was it identical with the 

 former. In the intersections of the circles with each 

 other I could perceive small lozenge-shaped forms 

 or minute patterns, but not clearly enough to make 

 them well out. On both these occasions the colours 

 were lively and conspicuous 



G 



