THE PRESIDENT'S ADDRESS. 281 



of the microscope be shortened a sensible amount, the amplifica- 

 tion is considerably reduced ; hence it is equally obvious, that 

 if the magnification has to be brought up again to what it 

 was before the tube was shortened, a stronger ocular must be 

 employed. To make this matter quite clear, I must point out that 

 there are two tube-lengths in the microscope, whether of English 

 or Continental length — the mechanical and the optical. The 

 former is measured from the shoulder of the objective to the 

 upper end of the draw-tube, where the eye-piece drops in, and is 

 often called the tube-length. Its length is 250 mm. or 10 in. 

 for the English type, and 160 or 170 mm. for the short or 

 Continental design. The optical tube-length, however, is alto- 

 gether different, and seldom heard of by the average worker. 

 To start from the commencement of the subject, you have all 

 heard of the focal planes of an objective. It is to the upper 

 focal plane of the microscopic objective that I w 7 ant to call your 

 attention for a moment. It would take too long to explain how 

 its position is easily found experimentally ; but for the sake of 

 simplicity, and to make what follows more readily understood, 

 I may mention that it usually lies near the front of the objective, 

 either in front or behind the object-glass. In two apochromatic 

 T T 2-in. objectives actually measured, I found it was 22-7 mm. below 

 the shoulder in one case, and 23'3 mm. in the other. This gives 

 you a sufficiently accurate idea of its whereabouts, and will enable 

 you to follow what I am going to say. If you were to place a 

 piece of ground glass in this upper focal plane, you would find — 

 perhaps to your astonishment — that the object was not magnified 

 at all. Placed at 4 mm. it would be amplified twice, whilst at 

 12 and 18 mm. it would be magnified six and nine times respec- 

 tively. You will see, then, that this distance away from the 

 upper focal plane' (whatever it may be), divided by the focus of 

 the objective, gives you the magnification of the object. It is 

 this distance that is called the " optical tube-length." It is per- 

 missible to define it as the distance at which the image is formed 



