74 The Microscope. 



Funke's ' Lehrbuch d Physiologie ') considered that parallel black 

 lines could be seen when the wotj of a millimetre apart (js^^ of 

 an inch)." 



" Passing from microscopic objects, we note that at a distance 

 of one foot a person with normal sight can scarcely see any ob- 

 ject less than.one-twent3^-fifth of an inch. At greater distances 

 the size must increase comparatively." 



This statement seems so much opposed to my experience that 

 I have submitted the following described tests to nineteen differ- 

 ent persons of ages ranging from seventeen to upwards of fifty 

 with the results given in the accompanying table. 



No. 1. consisted of a piece of black paper approximately one 

 mm. (one-twenty-fifth inch) square pasted upon a white back- 

 ground. No. 2. consisted of a similar white square upon a black 

 back -ground. No. 3. consisted of a black line approximately 

 one mm. broad upon a white back-ground. To be exact the di- 

 mensions of No. I. were 1.08 mm x 1.04 mm; No. II. 1.08 mm 

 X 1.98 mm. No. III. 1.12 mm in width. 



The manner of making the tests was to hang the cards in a 

 good light, not artificial, and approach them from such a dis- 

 tance that they were invisible, and note the respective distance at 

 which they became visible, and at which the shape of the squares 

 could first be defined. 



The accompanying table gives the results of several tests : The 

 first column gives the age, the second the condition of eyes when 

 known, the third the distance in feet and inches at which the 

 black square first became visible, the fourth the distance at 

 which the black square could be defined, the fifth and sixth 

 give the same particulars as t6 the white square, and the seventh 

 gives the distance at which the line first became visible. 



A large number of other tests were made with another set of 

 cards, but unfortunately one of the students lost them before 

 they had been measured, so that the results are not in the table. 

 There are however, a sufficient number of observations to show 

 pretty nearly the average limit of normal vision and to demon- 

 strate the gross inaccuracy of the above quotation from Dr 

 Tidy's work. 



3 In the experience of the writer, lines very much .smaller can be readily made 

 out by the average eye. A line ruled on glass or metal, which is less than 1,> in diameter 

 is distinctly visible. Profepsor Rogers states that he has seen lines one one hundred 

 and fifty thousandth inch in diiineter, and this with the unaided eye. Diftraction un- 

 doubtedly has much to do with this result. 



