RETENTION. 



sat'ions, and live retaining 1 the relicts of 

 them, seem to depend upon the same or- 

 ganic causes, whatever they be. In some 

 instances sensible changes perceptibly 

 continue after the sensible objects arc re- 

 moved. Two or three facts, which eve- 

 ry one must have noticed, or may notice, 

 will illustrate this principle. If a piece 

 of stick be burnt at one end, and the 

 lighted end be turned quickly round in 

 a circle, the luminous point will appear 

 to the eye as a complete luminous circle ; 

 the changes of the optic organs continu- 

 ing till the image of the luminous point 

 returns to any given point of the retina. 

 Again, the sensible changes produced by 

 sound, perceptibly continue after the ex- 

 ternal cause ceases. If a sounding body 

 be struck very rapidly with a stick, we do 

 not perceive any interval, and as Hartley 

 observes, the must simple sounds which 

 we hear, being reflected from the neigh- 

 bouring bodies, consist of a number of 

 sounds succeeding each other at different 

 distances of time, according to the dis- 

 tances of the reflecting bodies. The sen- 

 sible changes produced by the other 

 senses, also continue some time after the 

 impressions which have "been made up- 

 on them. If a hard body be pressed up- 

 on the palm of the hand, it is not easy to 

 distinguish, for a few seconds, whether it 

 remains or is removed. And tastes con- 

 tinue to be perceived long after the sapid 

 material is withdrawn. 



This play of the organs, (which how- 

 ever is rather to be referred to the ex- 

 ternal than to the mental organs), gives 

 rise, in the case of vision, to a number 

 of very singular and interesting pheno- 

 mena, by some philosophers called ocular 

 spectra. A considerable variety of them 

 are stated by Dr. R. Darwin, of Shrews- 

 bury, at the end of. the second part of 

 Darwin's Zoonomia. We shall select a 

 few of the most striking. 



Place about half an inch square of white 

 paper on a black hat, and looking steadi- 

 ly on the centre of it for a minute, re- 

 move your eyes to a sheet of white pa- 

 per ; after a second or two a dark square 

 will be seen on the white paper, which 

 will be seen for some time. A similar 

 dark square will be seen in the closed 

 eye, if light be admitted through the eye- 

 lids. So, after looking at any lumi- 

 nous body of a small size, as at the Sun, 

 for a short time, so as not much to fatigue 

 the eyes, this part of the retina becomes 

 less sensible to smaller quantities of light: 

 hence, when the eyes are turned upon 

 other less luminous parts of the sky, a 



dark spot is seen, resembling the shape 

 of the luminous body. To the smne 

 cause Dr. li. Darwin ascribes those dark 

 coloured floating spots, \\l,irh are easily 

 perceptible when the eyes are a little 

 weakened by faiigue, and during ill- 

 nesses which are attended with grc.at 

 debility. He says, that as these spec- 

 tra are most easily discernible when our 

 eyes are weakened by faiigue, it has 

 frequently happened that people of de- 

 licate constitutions have been much 

 alarmed at them, fearing a beginning 

 decay of their sight, and thence have 

 fallen into the hands of ignorant ocu- 

 lists. They are not, however, he ob- 

 serves, the preludes to any disease, and 

 it is only from our habitual inattention 

 to them that we do not see them on 

 all objects every hour of our lives. 

 As the nerves of very weak people, he 

 continues, lose their sensibility by a 

 small duration of exertion, it frequently 

 happens that sick people, in the ex- 

 treme debility of fevers, are perpetual- 

 ly employed in picking something off 

 from the bed clothes, owing to their 

 mistaking the cause of these dark 

 spots. An Italian artist, a man of strong 

 abilities, relates, that having passed tiie 

 whole night on a distant mountain, with 

 some companions and a conjuror, and 

 performed many ceremonies to raise the 

 devil, on their return in the morning to 

 Rome, looking up when the sun began 

 to rise, they saw numerous devils run on 

 the tops of the houses as they passed 

 along. So much were the spectra of 

 their weakened qyes magnified by fear, 

 and made subservient to the purposes of 

 fraud or superstition. 



Again, make with ink, on white paper, 

 a very black spot about half an inch in 

 diameter, with a tail about an inch in 

 length, so as to represent a tadpole. 

 Look steadily at this spot for about a mi- 

 nute, and on moving the eye a little, the 

 figure of the tadpole will be seen on the 

 white part of the paper, which figure will 

 appear whiter or more luminous than 

 the other part of the paper. This Dr. 

 R Darwin brings as one proof, that 

 when the retina has been subjected to 

 a less excitement, it is more easily 

 brought into action by being subjected 

 to a greater. A surface appears black 

 in consequence of its absorbing all the 

 rays of light ; that part of the retina, 

 therefore, which is unemployed while 

 looking at the spot, is afterwards more 

 sensible of the light from the white pa- 

 per, than those parts which had pre,vi- 



