530 Supplement to Dr. Parry's Essay 



I applied these principles in the following way. The microscope being so dis- 

 posed that I could look through it while sitting on a high seat, a piece of white paper, 

 smoothly pasted on a small cube of wood, was placed on the ground immediately 

 beneath. Having found that a proper degree of distinctness could not be obtained 

 by day-light, I made my examinations at night, the object being strongly illuminated 

 by the light of an Argand lamp reflected in the usual way from the concave spe- 

 culum below the stage of the microscope. The distance from my eye to the paper 

 below being now the greatest that I could reach with a pair of compasses at arm's 

 length while looking through the microscope, I found the image of the object pro- 

 jected on the paper to be magnified 1250 times. I then took a small lock of 

 scoured wool, consisting of 60 or 80 filaments ; and having gently strained it longi- 

 tudinally on a strip of glass, I waxed down both ends, so as to keep the filaments 

 immoveably fixed. The wool being thus disposed, the lower side of the glass was 

 blackened with Indian ink, except in certain narrow spaces exactly in the middle, 

 and at the distance of -|- an inch from each end of the lock, as represented in the 

 following figure. 



V 



The unblackcned spaces being then placed in the focus of the microscope, 10 fila- 

 ments in each were successively examined, and their images on the paper below 

 carefully measured with compasses on a scale exhibiting 40odths of an inch. The 

 magnifying power, as above stated, being 1250, if the projected image of a filament 

 measures ^^ of an inch, then -%-%■— i250i=:-y-oH§o-o of an inch: in decimals, 

 0,00084, and in a vulgar i''raciion -,-j-Vo of an inch. Or if the sum of 10 observa- 

 tions be VoV of a" i"ch, then VVV : 1250=^-0^0^00- or 0.020773 :=^J-5-j- of 

 an inch; which is the mean diameter of the 10 filaments in that part. Each lock 

 of wool being thus examined in three different parts, the mean of the three measure- 

 ments in decimals was divided by three for a mean diameter, which was reduced 

 into a vulgar fraction by the common method. In this mode, therefore, the liiieness 

 of each ciul, and of the middle of each specimen, is deduced from the mean dia- 

 meter of 10 filaments ; and that of the whole specimen from the mean diameter of 

 30 filaments. 



