ON INSTRUMENTS AND THEIR USE. 25 



be found double the brightness of the remaining 

 portions of the circles. 



We are indebted to stereoscopic vision for the 

 perception of relief or form in three dimensions, 

 which occurs when the images falling upon the 

 corresponding points of the two retinse are not 

 exactly similar. In looking at an object with both 

 eyes the rays do not run parallel from one side of 

 the object to the eye on that side, but the right eye 

 centres itself to the left side of the object and vice- 

 versa. This may readily be seen by holding up a 

 finger between our eyes and the wall, and looking at 

 the latter. Two fingers may be seen projected on 

 the wall, one of these is seen by the right eye and the 

 other by the left ; but our visual impressions do not 

 inform us which picture is formed by either eye in 

 particular. Now, while steadfastly looking at the 

 wall, close the right eye and the left finger will 

 disappear, while on shutting the left eye, the right 

 finger is rendered invisible. 



When two similar pictures are presented to the 

 eyes, the impression is more vigorous and looked 

 at with greater ease than when one eye only is 

 employed; vision in this case is called pseudoscopic. 



Binocular vision should be employed wherever 

 practicable ; it will be found much less trying to 

 the eyes than monocular efforts. 



It has now been shown that the human eye is 

 extremely liable to imperfections, and being so, strict 

 attention to details is demanded from the micro- 

 scopist. Now, although the human eye is such a 

 wonderful instrument, there are many problems it is 

 unable to solve without extraneous assistance. Take, 

 for example, the bunt of wheat, Tilletia faries. With 



