AND THE ULTIMATE POWER OF THE MICROSCOPE. 579 



to this. How far astronomy may afFonl us data which may be calcu- 

 lated with exactness, and applied to this subject, I Avill not discuss; 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- 

 lic 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 yig 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 « oVc '^^ ^ 

 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 upon a 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 ^^^ of a 

 line in thickness, by holding them towards the light, may be discerned 

 with the naked eye. Spider-threads measure ^^q to gTj'o o "^ ^ ^'°^ ' 

 threads of the silk-worm ^ Jo* ^^^ latter in the cocoon are double. 



The result of these inquiries is as follows : 



Optical instritments enable us to see with distinctness of coloured 

 sfjuare 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 -^j, to TTrmr of * hne in diameter, which are hindered 

 from uniting by the covering of grease. These jinest quicksilver globules are 

 not to be distinguished by the naked eye even in sunshine, but form with the 

 white lard a gray colour. L^irger globules may often be perceived in it. The 

 £ftt covering may probably dim the metallic lustre. 

 • Vol. 1.— Part IV. 2 r 



