ae 
4 
AND THE ULTIMATE POWER OF THE MICROSCOPE. 579 
to this. How far astronomy may afford us data which may be calcu- 
lated with exactness, and applied to this subject, I will not diseuss; for 
even if the measurement of the strength of light did not give a retar- 
dation, subject always to the prevailing theory of light and hypothetical, 
‘no certain conclusion can be deduced for the want of direct knowledge 
as well of the magnitude as of the distance of the heavenly bodies. 
Bodies which reflect light can indeed be examined, but the result of my 
experience has no particular bearing as to optical instruments. Metal- 
Jie lustre, which is a very powerful reflector, may, according to my 
experiments on gold dust, be discerned with the naked eye in common 
daylight up to the ;3, of a line, therefore double in proportion to 
colours. But this same bright surface when magnified 380 times, 
appears then dull and uneven, and the corpuscle acts only as a black 
one, or becomes transparent with a leek-green hue. The coloured 
transparency of gold seems to commence at the thickness of 35'55 of a 
line in diameter, and is evidently not the consequence of porosity. 
Fluid metals might probably form the smallest magnitudes attainable ; 
but even could we see with the microscope the reflected light of the 
multiples of the magnifying power with 100, &c., the outlines of bodies 
so minute would still remain the more undiscernible the stronger their 
reflection of light is in proportion to their magnitude. The dust of 
‘diamonds upona black ground in concentrated solar light, may probably 
afford the ultimate square power of reflected light; I have however 
hitherto not been able to make any observations on this point*. 
With lines it is very different. Non-transparent threads 71, of a 
line in thickness, by holding them towards the light, may be discerned 
with the naked eye. Spider-threads measure 33, to 37/55 of a line; 
threads of the silk-worm 53,. ‘The latter in the cocoon are double. 
The result of these inquiries is as follows: 
Optical instruments enable us to see with distinctness of coloured 
square parts of a line only multiples of their magnifying power with the 
number thirty-six, or at most, but then not distinctly, up to the number 
forty-eight. The limit for lines and light impressions lies much further, 
_and is determined by the intensity of the light. 
The most accurate microscope, with a distinct magnifying power of 
* I obtained the finest particles of gold by scraping gilt brass. By filing pure 
gold I always obtained much coarser particles. 
The thickness of leaf gold used for gilding is quite imperceptible to the naked 
eye, like the edge of a very sharp knife. If they are distinguished, it is by 
_ means of the inequalities of the bent border, and this is an optical deception. 
In the gray mercurial ointment the quicksilver forms, if well prepared, almost 
~ equal globules of about 345 to x55 of a line in diameter, which are hindered 
_ from uniting by the covering of grease. These finest quicksilver globules are 
not to be distinguished by the naked eye evenin sunshine, but form with the 
_ white lard a gray colour. Larger globules may often be perceived in it. The 
fat covering may probably dim the metallic lustre. 
Vou. I1.—Parrt IV. QR 
