316 



OPTICS. 



These spectra were always surrounded with a ring of 

 the accidental colour. If, when one of these spectra 

 is visible, we press the eye to one side, the spectrum 

 will appear to be absolutely immovable, if the experi- 

 ment is not made with much attention. It will be 

 found, however, by pressing both the ryes at once, 

 and by due attention to their corresponding motions, 

 that the spectrum does move, and that it is seen by 

 the eye in the same manner as if it were the image 

 of an external object, conformably to the law of vi- 

 sible direction. By means of pressure upon the eye- 

 ball, ocular spectra may be produced ; and when 

 spectra, produced by external impressions of light, 

 are seen by the eye, their colours are changed by 

 pressure on the eyeball. The pressure of the blood- 

 vessels on the back of the eye often produces spec- 

 tra, in particular states of the stomach. In slight 

 affections, these spectra are floating masses of blue 

 light, which appear and disappear in succession; but, 

 in severe ones, they become green, and sometimes 

 rise to yellow : hence it follows, that pressure upon 

 the retina creates the sensation of light and colours. 



Colours produced by the unequal Action of Light 

 upon the Eyes. If we hold a slip of white paper ver- 

 tically, about a foot from the eye, and direct both 

 eyes to an object at some distance beyond it, so as to 

 see the slip of paper double, then, when a candle is 

 brought near the right eye, so as to act strongly upon 

 it, while the left eye is protected from its light, the 

 left hand slip of paper will be of a tolerably bright 

 green colour, while the right hand slip of paper, seen 

 by the left eye, will be of a red colour. If the one 

 image overlaps the other, the colour of the overlap- 

 ping parts will be white, arising from a mixture of 

 the complementary red and green. When equal can- 

 dles are held equally near each eye, each of the 

 images of the slip of paper is white. If, when the 

 paper is seen red and green, by holding the candle 

 to the right eye, we quickly take it to the left eye, 

 we shall find that the left image of the slip of paper 

 gradually changes from green to red, and the right 

 one from red to green, both of them having the same 

 tint during the time in which the change is going on. 

 This experiment seems to confirm the observation of 

 Dr Brewster, that in certain highly excited states 

 of one eye, the reverse impression may be conveyed 

 from the one eye to the other. 



Insensibility of certain Eyes to particular Colours. 

 Various cases have been described, in which persons 

 capable of performing the most delicate functions of 

 vision are unable to distinguish particular colours, 

 and, what is certainly a most remarkable fact, this 

 imperfection runs in families. A shoemaker by the 

 name of Harris, at Allonby in Cumberland, could 

 only distinguish black and white. He was unable, 

 when a child, to distinguish the cherries on a tree 

 from the leaves, except by their shape and size. He 

 had two brothers whose perception of colours was 

 almost equally defective, one of whom constantly 

 mistook orange for grass green, and light green for 

 yellow. He had two other brothers and sisters, who, 

 as well as his parents, had no such defect. Another 

 case of a Mr Scott is described by himself in the 

 Philadelphia Transactions for 1778. He did not 

 know any green colour ; a pink colour and a pale 

 blue were perfectly alike to him. A full red and a 

 full green were so alike, that he often thought them 

 a good match ; but yellows, light, dark, and middle, 

 and all degrees of blue, except pale sky-blue, he 

 knew perfectly well ; and he could discern, with 

 particular niceness, a deficiency in any of them. 

 A full purple and a deep blue, however, sometimes 

 baffled him. Mr Scott's father, his maternal uncle, 

 and one of his sisters, and her two sons, had all the 

 same defect. Mr Dugald Stewart, Mr Dalton, and 



Mr Troughton, arc examples of the same inability to 

 distinguish certain colours. Mr Harvey lias de- 

 scribed, in the Edinburgh Transactions, the case of a 

 tailor, then alive, and aged sixty, who could distin- 

 guish with certainty only white, yellow, and gray. 

 On one occasion, he repaired an article of dress with 

 crimson in place of black silk ; and. on another oc- 

 casion, he patched the elbow of a blue coat with a 

 piece of crimson cloth. He regarded indigo and 

 Prussian blue as black, purple as a modification of 

 blue, while green puzzled him extremely. He con- 

 sidered carmine, lake, and crimson to be blue. The 

 solar spectrum appeared to him to consist only of 

 yellow and light blue. None of the family of this 

 person had the same defect. In these various (ases, 

 the persons are insensible to red light, and all the 

 colours into which it enters. Mr Dalton thinks it 

 probable that the red light is, in these cases, absorbed 

 by the vitreous humour, which he supposes may 

 have a blue tint. If, which is probable, the choroid 

 coat be essential to vision, we may ascribe the loss of 

 red light in certain eyes to the retina itself having a 

 blue tint. If the dissection of the eye of any person 

 who possesses this peculiarity shall not establish 

 either of these two suppositions, we must content 

 ourselves with supposing that the retina is insensible 

 to the colours at the end of the spectrum, just as the 

 ear of certain persons has been proved by doctor 

 Wollaston to be insensible to sounds at one extre- 

 mity of the scale of musical notes, while it is per- 

 fectly sensible to all other sounds. 



Optical Instruments. The impediments to the 

 vision of very near objects arise from too great a 

 divergence of the rays in each pencil incident on the 

 eye, and are remedied by the microscope. The most 

 powerful single microscopes are very small globules 

 of glass, which any person may make for himself, by 

 melting the ends of fine threads of glass in the flame 

 of a candle ; or by taking a little fine powdered glass 

 on the point of a very small needle, and melting it 

 into a globule before a smooth blow-pipe. It was 

 with such microscopes as these that Leuwenhoek 

 made all his wonderful discoveries, most of which 

 are deposited in the British museum. The double or 

 compound microscope differs from the preceding in 

 this respect that it consists of at least two lenses, 

 by one of which an image is formed within the tube 

 of the microscope ; and this image is viewed through 

 the eyeglass instead of the object itself, as in the 

 single microscope. In this respect, the principle is 

 analogous to that of the telescope, only that, as the 

 latter is intended to view distant objects, the object- 

 lens is of a long focus, and consequently of a mode- 

 rate magnifying power, and the eyeglass of a short 

 focus, which magnifies considerably the image made 

 by the object-lens ; whereas, the microscope being 

 intended only for minute objects, the object-lens is 

 consequently of a short focus, and the eyeglass, in 

 this case, is not of so high a magnifying power. The 

 solar microscope is a kind of camera obscura, which, 

 in a darkened chamber, throws the image on a wall 

 or screen. It consists of two lenses fixed opposite a 

 hole in a board or window-shutter one which con- 

 denses the light of the sun upon the object (which is 

 placed between them), and the other which forms 

 the image. There is also a plain reflector placed 

 without, moved by a wheel and pinion, which may 

 be so regulated as to throw the sun's rays upon the 

 outer lens. Mr Adam's most ingenious invention 

 the lucernal microscope is also to be considered as 

 a kind of camera obscura, only the light, in this lat- 

 ter case, proceeds from a lamp instead of from the 

 sun, which renders it convenient to be used at ;ill 

 times. From what has been said on the nature of 

 the microscope, the principle of the telescope may be 



