On Tube Length. By Edward M. Nelson. 131 



in front of the back focus F'. The front surface and focus are repre- 

 sented by long lines, while the back surface and focus are indicated 

 by shorter lines. The tube A should be measured from F, a point 

 situated between the eye- lens and the field-lens, at a distance from 

 the eye-lens equal to one-fourth of the distance between the eye-lens 

 and field-lens. The tube length d is measured from E, a point lying 

 a littie further beyond the eye-lens than the diaphragm does in front 

 of it. 



The point F', the back focus of the eye-lens, is important, because 

 close to it lies the back focal point of the entire Microscope. This 

 point can be readily found by using the eye-piece as a simple lens and 

 focussing the window bars on a piece of paper, the field-lens being, 

 of course, turned towards the window. It is important that the point 

 F should be kept in a certain fixed position with regard to the 

 mechanical tube, and manufacturers should be more careful in this 

 respect. The great bulk of Microscopes have Huyghenian eye-piece3, 

 and the common practice of makers, both here and on the Continent, 

 is to let the eye- lens rest on the top of the tube ; the two exceptions 

 to this rule are Powell's, which have been ringed since 1839, and Zeiss' 

 new compensating Huyghenians. 



Every microscopist wishes to be able to determine the magnifying 

 power of his Microscope, by merely multiplying the initial power of 

 the objective by that of the eye-piece. 



Let c be the conventional distance of accommodation, p the power 

 of the entire Microscope, while N and n' are the initial powers of 

 the objective and eye-piece respectively ; then, as we have seen above, 



G G C 



F = ,f===, and/' = — ; inserting these values in the first equa- 

 tion F = =4? , we have p = •, so that, when A is made equal 



Ac 



to c, jp = N ri, the condition desired by every Microscopist. 



Now if we turn to old works on the Microscope, we shall find 

 various values assigned for c. Baker in 1743 makes c = 8 in. ; both 

 Brewster and Pritchard in 1837 make it equal to 5 in. ; but Pritchard 

 in 1838 and Ross in 1839 increase it to 10 in. So, too, if we examine 

 old Microscopes, we shall find corresponding differences in the length 

 of the mechanical tubes. Old non-achromatic Microscopes had short 

 objective mounts, but the eye-piece was screwed to the top of the body, 

 so that the value of A for all except very low powers was just about 

 8 in., and therefore the formula j) = N»' gave a fairly accurate idea 

 of the magnification. An early achromatic Microscope by Hugh 

 Powell (a compound and single Microscope, dating about 1839) had 

 a mechanical tube of 7^ in. ; this reduction of ^ in. allowed for the 

 increased length of the mount of the achromatic objective. A little 

 later we find that c is increased to 10 in. ; and then we meet with an 

 increase of 1 ^ in. in the mechanical tube ; the objective mount is also 

 lengthened to about the size it is at the present time ; and the eye- 



k 2 



