SCIENCE. 



'e, sun when near the horiaon. The field was about two 

 CurioMtie* n ,i| ea distant, and was divided into furrows, which 

 in ' wen- directed to the eye of the observer as shown in 

 pJTT^ ' F 'K- 15 > !>lale CCCCLXXXVf, where A B, C D, EK, 

 t CCCLXXXTL represent these furrows. These furrows are of course 

 rig. 15. depressed, and the growing corn rises gradually from 

 two adjacent ones towards the middle m, op, so that 

 the surfaces A m C, C o K were convex. The drills of 

 corn on the highest summit* mn, op, caught the rays of 

 the setting sun, which shone upon them very obliquely 

 in the direction S*> and illuminated the summits late- 

 rally, while the furrows A B, C I >, K I , "-re in sha- 

 dow. The consequence of this disposition of the light 

 and shade was, that the whole field seemed to be 

 trenched, and the corn to be growing in the trenches as 

 well as upon the elevated beds between them. The 

 half furrow A B n ru being shaded on its edge A B, and 

 illuminated on its edge m n, became the elevated part 

 of the trenched ground, while the other half m n C D 

 appeared the sunk part, in consequence of the side m n 

 being illuminated, and its other side C D in shade. At 

 a certain period of the day, this illusion did not take 

 place, and it was dispelled the moment the sun had 

 set. This very singular illusion 1 have seen on several 

 days in July. The telescope had no effect whatever 

 in producing it, as it showed objects erect. 



An illusion of an analogous nature I once observed 

 when looking at the abbey church of Paisley, where the 

 clustered columns of a Gothic pillar all sunk into hol- 

 low flutings. The cause of this deception I could not 

 at the time discover, but it must have arisen from some 

 mistaken notion respecting the direction in which, the 

 object was illuminated. 



The last species of illusion of this nature, and per- 



blue from pink by daylight ; and in the solar spectrum 

 the red is scarcely vuible, the rest of it appearing to L 

 consist of two colours, yellow and blur.. Dr. Batters, MP-,~V 

 in a letter addressed to the editor of this work, has de- 

 scribed the case of Mr. H. Tucker, ton of Dr 

 Tin k.-r of A>.titnirton, who mistakes orange for green, 

 like one of the Harrises. Like Mr. Dalton he could 

 not distingui-h blur from pink ; but he always knew 

 yellow. The colours in the spectrum lie dencribe* a 

 follows i 



1. Red Mistaken for . brown, 



2. Orange . K Tetn > 



3. Yellow, generally known, but sometimes 



taken for . - orange, 



4. Green mistaken for . . orange, 



5. Blue ... . . P'nk, 

 fi. Indigo . . . purple, 

 7. Violet . . . purple. 

 Mr. Harvey has described, in a paper read before the 



Royal Society of Edinburgh, the case of a person now 

 alive, and aged 60, who could distinguish with cer- 

 tainty only white, yellow and grey. He could, how. 

 ever, distinguish blues when they were light. Dr. 

 Nichols mentions a case where a person who was in 

 the navy purchased a blue uniform coat and waist-coat, 

 with red breeches to match the blue, and he has men- 

 tioned one case in which the imperfection is derived 

 through the father, and another in which it descend- 

 ed from the mother. 



In the case of a young man in the prime of life, 

 with whom the writer of the article is acquainted, 

 only two colours were perceived in Dr. Wollaston's 

 spectrum of five colours, viz. red, green, blue and 

 violet. The colours which he saw were blue and 



haps the most remarkable of all of them, may be pro- Qfu O r yellom, as he did not distinguish these two 

 duced by a continued effort of the mind 1 deceive it- from Qne another When a] j the Colour8 o f t he spec- 

 self. If we take one of the intaglio moulds us. trum were absorbed by a re daish glass, excepting red 

 making the bas-reliefs of that able artist Mr. B ung, flnd daff . ^ reen> b e 8aw on ] y one co l our> v j z . yellow or 

 i ,i; M . .,- <>,> ;> .t.,iii v without notic orange. When the middle of the red space - K 



f>n the in- 

 visibility of 

 certain co- 

 lours to 

 certain eyes. 



and direct the eye to it steadily without noticing 

 rounding objects, we may coax ourselves into the be- 

 lief that the intaglio is actually a bas-relief. It is dif 

 ficult at first to produce the deception, but a mtle 

 practice never fails to accomplish, k. 



I have succeeded even in carrying this deception so 

 far as to be able, by the eye alone, to raise a complete 

 hollow mask of the human face into a raised head. In 

 order to do this, we must exclude the vision of other 

 objects ; and also the margin or thickness of the cast. 

 This experiment cannot fail to produce a very great 

 degree of surprise in those who succeed in it ; and it 

 will no doubt be regarded by the sculptor who can use 

 it as a great auxiliary in his art. See Dr. Brewster'a 

 Journal/ of Science, No. VII. 



4. On the invisibility nf certain colours to certain eyes. 



Several cases have been recorded in the Philosophical 

 Transactions where persons with sound eyes, capable 

 ^ P e rf rm i n {? all then: ordinary functions, were inca- 

 pable of distinguishing certain colours, and what is still 

 more remarkable, this imperfection runs in particular 

 families. Mr. Huddart mentions the case of one 

 Harris who could only distinguish black and white, 

 and he had two brothers almost equally defective, one 

 of whom always mistook orange for green. Another 

 ease is recorded in the Philosophical Transactions in 

 which full reds and full greens appeared alike, while 

 yellows and dark blues were very easily distinguished. 

 Our celebrated chemist ."Mr. Dalton cannot distinguish 



\Vhcn the middle of the red space was ab- 

 sorbed by a blue glass, he saw the black line with 

 the yeliovr on each side of it. We are acquainted 

 with another gentleman who has a similar imperfec- 

 tion. 



In all the preceding cases, there is one general fact, 

 that red lighl, and colours in which it forms an ingre- 

 dient, are not distinguishable by those who potseif 

 the peculiarity in question. Mr. Dalton thinks it pro- 

 bable that the red light is, in these cases, absorbed by 

 the vitreous humour having a blue colour; but as this 

 is a mere conjecture, which is not confirmed by the 

 mo3t minute exaniinntion of the eye, we cannot hold 

 it as an explanation of the phenomena. Dr. Young 

 thinks it much more simple to suppose the absence or 

 paralysis of those fibres of the retina which are cal- 

 culated to perceive red, while Dr. Brewster conceive* 

 that the eye is in these cases insensible to the colour? 

 at one end of the spectrum, just as the ear of certain 

 persons has been proved, by Dr. Wollaston, to be in- 

 sensible to sounds at one extremity of the scale of mu- 

 sical notes, while it is perfectly sensible to all other 

 sounds. 



If we suppose, what we think will ultimately be de- 

 monstrated, that the choroid coat is essential to vision, 

 we may a.-cribe the loss of red light in certain eyes to 

 the retina itself having a blue tint. If this should be the 

 case, the light which falls upon the choroid coat will be 

 deprived of its red rays, by the absorptive power of the 

 blue retina, and consequently the impression convey-- 



