THE MAGNIFYING POWER OF THE EYE. 595 



while the remainder pass on through the pupil, which 

 communicates to the image formed on the retina its 

 circular form. 



" For the purpose of presenting very small objects 

 mounted on microscopic slides in the usual way before 

 the eye, and to exclude the surrounding rays of light, 

 take an upright box of pasteboard about one inch and 

 a half deep, and an inch and a quarter in diameter (two 

 pill-boxes joined will do very well) ; cut a couple of slits 

 through one of its sides, sufficiently large to admit of the 

 slips of glass sliding to and fro. Make two apertures 

 (perforations with a needle) opposite to each other the 

 first the l-30th of an inch, and the second the l-40th of 

 an inch in diameter; 1 let these be so disposed, that when 



Fig. 330. 



the glass slip, with a small object mounted on its centre, 

 is introduced through the slits, the two apertures and the 

 object shall all correspond, and be in one straight line, 

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

 smaller opening, as in fig. 330. The image becomes more 

 distinct when more than one aperture is used; for the 

 intensity of light by which it is illuminated is thereby 

 increased, being almost in a direct ratio with the number 

 of the openings which are employed. 



(1) Sewing-needles are ordinarily sold in papers, numbered from 1 to 12, ac- 

 cording to their thickness. The diameters of apertures made with needles from 

 the papers marked Nos. 6, 7, 8, 9, and 10, when measured with the micrometer, 

 are equal to the l-36th, the l-38th, the 1-4-ith, the l-50th, and the l-70th of an 

 inch. 



QQ2 



