Dr. Goring on Achromatic Microscopes, 433 



the whole for this object, and is, indeed, absolutely neces- 

 sary to give the Amician reflector the power of showing this 

 object advantageously, though I have managed to see every 

 thing there is to be seen about a foot, without its assistance to 

 the effect of the said instrument. I may remark, that, being 

 a very sombre object, it is peculiarly calculated to be seen 

 unfavourably in the dark image of the reflector ; neverthe- 

 less, every point can be shown by the Amician metals of 0*6 

 focus and 0*3 aperture completely well, and with as much 

 ease too as by any other instruments greatly superior to 

 them in mere light or brilliancy of eflTect. A dark ground 

 of some kind or other is absolutely necessary to exhibit this 

 object in its proper relief ; a white one confuses it altogether ; 

 the power may vary from ^-^ to -^^ inch : candle or lamp light, 

 I think, does best for it, and the darker the apartment is 

 made in which we observe it the better, for darkness causes 

 the iris of the eye to expand, and receive a stronger impres- 

 sion from the faint rays issuing from such dull objects. 



A human hair is a beautiful proof object ; I repent that I 

 have not given a figure of it. 



Mr. W. Tulley has discovered a singular property in almost 

 all hairs, which I do not think has ever been remarked or de- 

 scribed. If a hair is drawn between the finger and thumb 

 from the end to the root, it will be distinctly felt to give a 

 greater resistance and a different sensation to what is expe- 

 rienced when drawn the opposite way ; in consequence, if 

 the hair is rubbed between the fingers it will only move one 

 way, (travelling in the direction of a line drawn from its ter- 

 mination to its origin from the head or body,) so that each 

 extremity may thus be easily distinguished even in the dark 

 by the touch alone. 



The mystery is resolved by the microscope. A hair viewed 

 on a dark ground as an opaque object, with a high power not 

 less than that of a lens of ^V ^^ ^^ i"^^ focus, and dully illu- 

 minated by a cup, which seems to answer best, is seen to be 

 indented with teeth somewhat resembling those of a coarse 

 round rasp, but extremely irregular and rugged ; as these 

 incline all in one direction, like those of a common file, viz. 

 from the origin of the hair towards its extremity, it suflfi- 



