POWER OF SHORT SPACES. 39 



but material combinations which serve to elucidate funda- 

 mental principles in optics by direct experiment ? 



When one of these apertures, only the l-200th of an inch 

 broad, is brought close to the eye, its apparent size is about 

 two inches. This is easily proved by observing that the 

 breadth of its image covers that of a line two inches long, 

 held up for the purpose of comparing the two at an interval 

 of ten inches, the distance at which we are accustomed to 

 view ordinary objects in order to gain an idea of their sup- 

 posed extension in space, and so to guess at their real magni- 

 tude. If this distance of ten inches were always preserved, 

 and if surfaces whose real dimensions are required were always 

 compared with a scale held at such a distance, the eye might 

 become instructed to appreciate relations of magnitude witli far 

 greater accuracy than it has hitherto attained. 



The comparison of the image of a very small object in 

 close proximity to the eye, with that of any larger object at 

 the usual distance for distinct vision, thus affords a correct 

 method of measuring the apparent increased magnitude of all 

 small bodies ; and it cannot be too strongly impressed on the 

 mind, that on looking through any aperture, whether small or 

 great, it always appears as large as all we see through it. 

 This has been happily expressed by an eminent writer. "If 

 you shut one eye and hold immediately before the other a 

 small circle of plain glass, of not more than half an inch in 

 diameter, you may see through that circle the most extensive 

 prospects, lawns and woods, and arms of the sea, and distant 

 mountains. You are apt to imagine that the visible picture you 

 thus see is immensely great and extensive ; but it can be no 

 greater than the visible circle through whicli you see it. If, 

 while you are looking through the circle, you could conceive 

 a fairy hand and a fairy pencil to come between your eye and 

 the glass, that pencil might delineate upon that little glass the 

 outlines of all those extensive lawns and woods, and arms of 

 the sea, and distant mountains, in the dimensions in which 

 they are seen by the eye." 



Since this was penned, the fairy hand and the fairy pencil 

 have both been actually discovered in the beautiful art of 

 photography. 



2. The extremities of the aperture appear rounded or semi- 

 circular. — We have seen how a circular perforation considered 

 as a radiant point admits a divergent pencil of rays, the cir- 

 cular base of which forms a large round disc or image at the 

 bottom of the eye (fig. 10). Now as a line mathematically 

 considered is made up of a number of points, so a transparent 

 line may be assumed to consist of a number of radiant points. 



