8 GORHAM, ON THE 



enlarged image, whilst the second prevents a too great con- 

 traction of the pupillary opening. Hence the necessity for 

 examining objects through small darkened tubes, and hence, 

 too, the necessity for closing the eye which is not engaged in 

 exploring. 



Again, we must not overlook the fact, that in using a small 

 aperture for the purpose of examining any transparent sub- 

 stance there are two methods which may be employed. By 

 the one, the object is viewed through the aperture ; by the other, 

 the aperture is viewed through the object. The former has been 

 almost always adopted by the curious, the latter scarcely ever. 

 It is capable, however, as these papers show, of eliciting so 

 many phenomena peculiar to itself, that I am surprised it has 

 not been frequently used, and the results carefully investigated. 

 Each of these plans throws a different picture on the retina 

 of the eye, and of this the transparent animal membrane chosen 

 for the following experiments will afford, when examined in 

 both ways, abundant exemplification. 



For the purpose of presenting very small objects, mounted 

 on microscopic slides in the usual way, before the eye at 

 small distances behind a minute aperture, and to exclude the 

 surrounding rays of light, I took an upright box of pasteboard 

 about one inch and a half deep, and one inch and a quarter in 

 diameter, and having cut a couple of slits through one of its 

 sides sufficiently large to admit of a slip of glass an inch 

 broad sliding to and fro, I made two small apertures opposite 

 to each other, the first the one-thirtieth of an inch, and the 

 second the one-fourth of an inch in diameter ; and these were 

 so disposed, that when the glass slip with a small object 



Fig. 34. 



mounted on its centre was introduced through the slits, the 

 two apertures and the object were all in one straight line ; 

 while the slide was about a quarter of an inch behind the 

 smaller opening, see fig. 34. 



