TJie Hclmholtz Theory of the Microscope. By J. W. Gordon. 417 



formed of parallel threads, arranged side by side or drawn in black 

 and white lines upon paper. Standing at such a distance from the 

 grating that with full accommodation of the eye, he can with 

 suitable spectacles just distinguish the lines of the grating from 

 one another, the observer places in front of his eye a card in which 

 several small holes of various diameters have been pierced and 

 judges whether he can through these holes still see the lines of the 

 grating and see them as well as without the card. The illumination 

 of the grating must be very bright; one printed on paper, for example, 

 may be illuminated by direct sunlight in order to secure the 

 necessary brightness of the object as seen through the opening. 



" By means of such an experiment I find that in fact a noticeable 

 deterioration of the image is produced by an opening of 1 ■ 72 mm. 

 diameter. This is much more pronounced if still narrower open- 

 ings are employed. 



" The grating can be replaced by a printed page, under suitable 

 conditions of use, that is to say, if it be placed at such a distance 

 from the observer that he can just read it perfectly. Then, if he 

 views it through an opening of about 1 mm. diameter, he will find 

 that it is difficult or even impossible to read it. But I find this 

 experiment less delicate than that with the grating. 



" Of course, in the making of these experiments care must be 

 taken to secure the best accommodation of the eye ; if that be 

 imperfect it may happen that the interposition of the card will 

 diminish the size of the diffusion circles on the retina, and so 

 improve the image." 



There are two criticisms which must here be interposed in 

 reference to this experiment. First, that it is an experiment upon 

 the power of the eye to discriminate small parts of an image, and 

 not upon the state of resolution of the image formed in the eye, 

 If I shut my eyes and put my hands upon a number of coins lying 

 side by side on a table I can count them with ease if they are half- 

 crowns, because the individual coin is much larger than my finger 

 tips. But if they are threepenny-bits, that is to say, just about 

 the same size as my finger-tips, I have the greatest difficulty in 

 counting them, and I suppose that if they were the size of Maundy 

 pennies I could not count them at all. I should not know whether 

 one, two, or three were under one finger at one time. 



The same kind of difficulty arises when we try to estimate by 

 sense impressions produced upon an organ like the retina — where 

 nerve-ends are distributed, as in the finger-tips, at finite distances 

 apart — the absolute state of resolution in an image formed upon 

 the retina. It is nothing to the point that the power of discrimi- 

 nation in the retina is many hundred times greater — in the sense 

 of being more subtle — than in the finger-tips. Notwithstanding 

 its great refinement, the retina has a limited power of discrimi- 

 nation, and it may well be that an image, which to the eye appears 



