The Microscope. 73 



It is thus impossible to do justice to the subject in a magazine 

 article, and those who desire further information, will find it in 

 the following works, of which the first two will be found most 

 generally useful. 



The chemistry of coal tar colors, Benedikt and Knecht ; London : George 

 Bell and Sons, 1886. Organic Analysis, by Albert B. Prescott. New York : 

 T>. Van Nostrand 1887. Commercial Organic Analysis, by Alfred H. Allen. 

 Vol. III. J. and A. Churchill, London, Eng., 1889. Tabellarische Uebersicht 

 der kunstlichen organischen Farbstoffe, Schultzand Julius ; R. Gaertner, Ber- 

 lin, 1888. Fortschritte der Theerfarbenfabricatiou und verwandter Industrie- 

 zweige, 1877-1887, Dr P. Friedlander; Julius Springer, Berlin, 1888. Hand- 

 buch der organischen Chemie, Dr F. Beilstein ; Leopold Voss, Hamburg and 

 Leipzig, Zweite Ausgabe, 1887. 



SOME EXPERIMENTS TO DETERMINE THE LIMIT OF 



VISION AS RELATED TO THE SIZE OF THE 



OBJECT OBSERVED.^ 



PROFESSOR M. D. EWELL, M. D., LL. D. 



IN Vol. I of Tiddy's Legal Medicine, page 248, we find the 

 following statements : 



" With respect to the smallest objects recognizable by the un- 

 assisted sight, there has been much difference of opinion. Car- 

 penter states (apparently on the authority of Ehrenberg)', that 

 the smallest square magnitude, black or white, which can be seen 

 on a ground of the reverse color, is about the 4^ to the ^io of an 

 inch, whilst particles that powerfully reflect light, such as gold 

 dust of the rrrs of an inch, can be seen with the naked eye in 

 common daylight. Bergman found that black and white check- 

 ers of ^V an inch square could be discerned at such distance that 

 the retinal image of each square could not have exceeded half 

 the diameter of one the cones of the bacillary layer, which are 

 said to have a diameter of uVg of an inch". Dr Vincent de 

 Gueret (of Creuse) in ' La France Medicale,' (No. 57 for 1875,) 

 states that objects to be seen at all must have a diameter of the 

 ^-isu of an inch. 



" Lines are more easily perceived than points. Thus opaque 

 threads of the Ww of an inch (L e. about half the diameter of a 

 silk worm's fibre) can be discerned by most people with the 

 naked eye when held towards the light^. Volkmann (quoted in 



1 Read before the ^m. Soc. Micrwicopists. 



2 This compounds to a distance of 27.77 from the eye. 



