1892.] MIOKOSCOPICAL JOURNAL. 93 



M.ignifyiiig Power of Objectives. 



By H. L. TOLMAN, 



CHICAGO, ILL. 



The question of how much a given objective will magnify has 

 always been an important but difficult one to answer, and every 

 assistance offered toward solving it is worthy of attention. The 

 distance often inches has been assumed as the proper interval be- 

 tween the objective and eye-piece, that being the average focal 

 length of the normal eye, but where to measure from and to is 

 not so easy to ascertain. Some opticians estimate from the end 

 of the tube of the microscope, others from the outside of the back 

 lens of the objective, others from a point midway between the 

 different lenses of the objective, and still others from the front end 

 of the latter. The difference between these two extremes is fully 

 two inches, which may cause a difference of 20 per cent, in the 

 results. Still others choose the point where parallel rays sent 

 through the lens from the front would come to a focus, called the 

 posterior principal focus, and one or two others, the posterior con- 

 jugate focus, still higher up the tube, a point where rays meet 

 which emanate from another point in front of the objective, at a 

 distance such that the size of the object and image are made equal. 

 This is an easily established place, but a theoretical consideration 

 of the optical principles involved shows that the only proper posi- 

 tion from which to measure the tube length is from the posterior 

 principal plane of the objective. In a simple lens this is easily 

 ascertained, and in a very thin lens can be called the centre of the 

 lens, but in a complex combination where the distance from the 

 front of the front lens to the back of the back lens is sometimes 

 two inches the exact point from which to estimate tube length 

 becomes important. These principal planes in nearly all converg- 

 ing lenses are situated inside the objective at different distances 

 from the centre of the combination, depending on the power of 

 the lens and the way in which the corrections are made. In two- 

 system objectives where the magnifying power is effected nearly 

 equally by both systems, the principal planes are near the centre 

 of the systems, while in some high-power objectives they may 

 cross one another, the posterior plane being in front of the ante- 

 rior. The principal foci anterior and posterior of a lens are also 

 two important points to know, and when these four data are given 

 they are all that are necessary for a discussion of the properties of 

 a lens. 



In the last journal of the Royal Microscopical Society under 

 the head of Measurement of Lenses, Prof. vS. P. Thompson has 

 given a very exhaustive and able article on how to ascertain this 

 point or plane from which the ten-inch tube length is to be meas- 

 ured. The instrument he uses is very complex and expensive, 

 but the measurements can be made, except for high-power objec- 

 tives, with a near approximation by anyone with a little mechan- 



