174 ON THE SENSES. 



tion being paid to tlie absence of their sensations ; and altbougb we dream of 

 scents and of tastes, it is not with the same necessity and constancy with which 

 we dream of objects of sight. Even the auditory nerve may long be at rest 

 without consciousness on Our part of the stillness, if no sensational momentum 

 summons the attention to watchfulness ; nor is it at all di£ferent as regards the 

 sense of touch. Hence, because we are accustomed to hold uninterrupted inter- 

 course with the outer world by means of the eye, and constantly to derive from 

 its perceptions the incentives to thought and action, the perception of a failure 

 of the visual power is unavoidable even to the parting consciousness ; hence the 

 cry of the poet : " Light ! more light ! " as the eyes with which he had looked 

 through nature and the human heart were extinguished in death. The construc- 

 tion which I have put on the dying words of Goethe is, I think, clear and con- 

 sistent, and I have placed it at the head of this essay for the purpose of impress- 

 ing more deeply on the reader the inestimable value of the sense whose nature 

 and office it will be the aim of the following pages to explain. 



There is no one probably who has not, at some time or other, propounded to 

 himself the question, which of the senses he would be most loath to dispense 

 with or to lose. The blind would answer, the sight; the deaf, the hearing; be- 

 cause each knows from experience, and is prone to overrate the disadvantages to 

 which the loss of either sense subjects him ; but he who, being possessed of all 

 the senses, seriously asks of what avail is each to him, will unquestionably con- 

 cur in opinion with the blind. No one would in such case think for a moment 

 of the sense of smell, nor would the most pampered palate be suffered to prefer 

 its pretensions in behalf of that of taste ; rather would we claim an exception 

 in favor of the sense of touch. But enough ; the eye is pre-eminently the jewel 

 of the organs of sense ! Let us seek to penetrate into its wonderful conformation. 

 To lay before the novice an intelligible representation of the office, mechanism, 

 and action of the eye, is, it must be confessed, a peculiarly difficult task, more 

 difficult than a like undertaking with regard to the ear, for the reason, in part, 

 that the learning connected with the proposed discussion is necessarily much 

 more comprehensive than that which regards the other organs. In order to 

 perform satisfectorily what we propose, it will be necessary to premise a careful 

 anatomical analysis of at least the most important parts of the complicated ap- 

 paratus of the globe of the eye, as well as the physical doctrine of the nature 

 of light; the laws of its propagation, refrangibility, reflection, &c.; the relations 

 of the separate parts of the eye to the light should be specially examined, and 

 finally the origin of the sensations of light and the modifications of vision are 

 to be considered ; the whole to be couched in terms and in a form intelligible to 

 the laity. We shall lose no time in a further detail of the difficulties or extent 

 of the undertaking, but it is proper to say that no exhaustive discussion of these 

 subjects is here contemplated ; we shall be content to have explained to our 

 readers v/hat is most essential, and to have imparted correct views of the most 

 important particulars. 



The eye fully corresponds in its arrangements and its purpose to the so-called 

 camera obscura, an instrument which we may safely assume to be generally 

 known, especially in view of its customary employment in the service of photo- 

 graphy. This instrument is designed to throw a distinct image of an object 

 situated before it upon a surface placed in its background, whether this surface 

 be a pellucid plate of glass on which we see the image as if it were painted on 

 pa,per, or a sensitive sheet treated with collodion or iodized silver plate, as is 

 practiced by photographers and daguerreotypists. In this last case, the image 

 is fixed, by the chemical influence of the rays of light which produce the various 

 parts of the figure, on the sensitive substance with which the plate is charged; 

 how this takes place,^ we need not here explain. 'V7hoever has seen the image 

 of a landscape or person on the glass plate of the camera obscura, will re-, 

 member that it is distinguished by two circumstances from the object it repre- 



