42 



CONSTRUCTION OF THE MICROSCOPE. 



band being composed of parallel lines of some fraction of a Prussian 

 inch apart; in some they are 1-lOOOth, and in others only l-4000th of 

 a Prussian inch separated. The distance of these parallel lines forms 

 part of a geometric series : 



0-001000 lines. 

 0'000857 „ 

 0-000735 „ 

 0-000630 „ 

 0-000540 „ 



0-000463 lines. 

 0-000397 „ 

 0-000340 „ 

 0-000292 „ 

 0-000225 „ 



To see these lines at all, it is requisite to use a microscope with a 

 magnifying power of 100 diameters ; the bands containing the fewest 

 number of lines will then be visible. To distinguish the finer lines, it 

 will be necessary to use a magnifying power of 2000, and then the lines 

 which are only l-47000th of an inch apart will be seen as perfectly 

 traced as the coarser lines. Of all the tests yet found for object-glasses 

 of high power, these would seem the most valuable. These tracings 

 have tended to confirm the undulating theory of light, the difierent 

 colours of the spectrum being exhibited in the ruled spaces according 

 to the separation of the lines ; and in those cases where the distances 

 between the lines are smaller than the length of the violet-coloured 

 waves, no colour • is perceived ; and it is stated, that if inequalities 

 amounting to -000002 line occur in some of the systems, stripes of 

 another colour would appear in them. 



Achromatic object-glasses for microscopes are of various foci, dif- 

 fering from 2 inches to 1-1 6th of an inch. 



Magnifying Power of Mr. Boss's Ohject-Glasses with his various 

 Eye-Pieces. 



