134 



SCIENTIFIC RECREATIONS 



There is a dark spot in every human eye that is, a spot which is 

 insensible to light. The eye is generally regarded as a perfect instrument, 

 but it is not yet so by any means. One of our great philosophers remarked 

 that if an instrument were sent home to him so full of errors he would feel 

 justified in returning it to the optician. But the eye has its dark place, the 

 punctum ccecum, and it can be discovered by covering the left eye with the 

 hand, and holding the present page at arm's length with the other. Then 

 fix the gaze on the small cross in the picture, and bring the book close up. 

 At a little distance the white ball will disappear from the page (fig. 139). 



The illustration (fig. 1 40) shows us a very curious optical illusion, and 

 one very easy to practise. Roll up a sheet of paper, and look through it, as 



Fig 140. An optical illusion. 



through a telescope, with the right eye, keeping both eyes open. Then 

 place the left hand open palm towards you against the roll of paper, you will 

 then appear to be looking through a hole in your left hand. Sometimes the 

 effect is produced without holding up the other hand to the roll, as shown in 

 fig. 140. 



Among optical illusions there are a great number that may be produced 

 by means of mirrors. The divided telescope is an example. The. apparatus, 

 raised on a firm stand, allows of one apparently seeing an object through a 

 stone or other opaque object, as shown in fig. 141. The illustration shows 

 the arrangement of the apparatus. The observer, looking through it, plainly 



