AT DIFFERENT DISTANCES. 21 



Professor Miiller has well discussed the whole of this subject,* and 

 the additional observations which have been made since the period at 

 which his account was published, consist less of new hypotheses than 

 of arguments and fresh facts tending either to support or controvert 

 one or other of the several explanations considered at length by him. In 

 further refutation, for example, of the doctrine according to which this 

 power of adaptation of the eye is attributed to an elongation of the entire 

 globe, effected by compression of it through the action either of the four 

 straight, or of the two oblique muscles of the eye, Hueckf states, that 

 owing to the firmness and resistance of the sclerotica in the perfectly fresh 

 eye, he was unable, even by considerable circular pressure, to produce any 

 appreciable elongation in the globe of fresh eyes from a bird and from 

 a cat; nor in consequence of such pressure did any remarkable alteration 

 in the distinctness of an image formed on the retina ensue. He en- 

 deavoured also to ascertain the effects of such pressure on the eye of the 

 living human subject, and for this purpose hollowed out a piece of cork, 

 and adapted it to the globe of the eye in such a manner that he could 

 thereby compress it against the inner wall of the orbit : and the result of 

 such compression was the production of but a slight change in the distance 

 of distinct vision. 



The increased convexity of the cornea, which was said to be one of the 

 important changes effected by compression of the eye, and on the occur- 

 rence of which its power of adaptation to the perception of near objects 

 was supposed to depend, could not be detected by Hueck. He atten- 

 tively watched the cornea while the sight was changed from an object 

 thirty feet distant from the eye to one only seven inches distant, but be- 

 yond the movements resulting from respiration and from the pressure of 

 the orbicularis muscle, he could not perceive any change in the cornea ; no 

 protrusion, and no sinking. This agrees with the observations of Dr. 

 Young, who also was unable to perceive any such change as was said by 

 Sir E. Home and others to take place. $ 



Another mode in which the action of the recti muscles was supposed to 

 aid the eye in adapting itself to the distinct vision of objects at different 

 distances was by retracting the globe and compressing it against the 

 posterior part of the orbit, whereby the axis of the eye would, it was sup- 



* Physiology, pp. 1136-50. 



f Ueber die Bewegung der Krystallinse, 1839, noticed by M. Tourtual in his Report on 

 the Progress of the Physiology of Vision. Miiller's Archiv. 1842, p. iii. 



J Miiller's Physiology, p. 1143. Mr. Smee (Vision in Health and Disease, page 16), 

 acting upon the suggestion made by Professor Miiller, has watched the images formed by 

 reflection on the cornea, and states that when the eye looks at distant objects these 

 images become smaller than when it is directed to near objects ; if this change in size really 

 ensues (though it is difficult to make quite sure about it) it would certainly seem to indi- 

 cate that the convexity of the cornea undergoes some alteration at such times. 



