POWER OF SHORT SPACES. 



41 



Bearinof in mind that the image of a linear aperture is not 

 a line but a plane, and that this can be revolved by inserting 

 it in the distal end of the dmscope, it will be seen that if the 

 object chosen for examination be a similar linear aperture 

 held close to the eye we obtain a second plane, the first of 

 which can be revolved in front of the second, and so the two 

 can be made to intersect at any ansfle. 



In order to illustrate this, let the planes P and p (fig. 12), 



Fig. 12. 



about two inches apart, be inserted at the ends of a darkened 

 tube, and let a small linear aperture, a and d, be made in each 

 of them. Now by revolving the plane p, the one aperture 

 will intersect the other. When common diffused light is 

 admitted through the further aperture d, the greater part 

 is intercepted in its passage towards the eye at E by the 

 plane P, but that which is transmitted will partake of the 

 form of the luminous space produced by the intersection of 

 the two. Thus when the apertures cross at a right angle, as 

 shown in the figure, the image which meets the eye is a square, 

 while it is rhombic at all other angles. 



This may be further illustrated by cutting two oblong 

 pieces, exactly similar in shape and size, from the lid and 

 the bottom of a common pill-box. 

 When the former is revolved upon 

 the latter, the quadrangular planes 

 to which we have referred are easily 

 imitated (fig. 13). 



Hence by holding two very nar- 

 row, linear apertures before the eye, 

 and examining them by diffused 

 light, all idea of mere linear exten- 

 sion is lost, and we obtain images 

 of the square and all possible varie- 

 ties of the rhomb. 



It is worthy of notice that such planes do not differ in form 



Fig. 1.3. 



