THE 'BODY' OF THE MICROSCOPE 159 



What was formerly known as the English standard tube had an 

 optical length for high and moderate power objectives of ten inches ; 

 with low powers, however, it was less. The mechanical tube-length 

 was 8 1 inches. 



Professor Abbe, in constructing his apochromatic objectives for 

 the English body, has taken the mechanical tube -length at 9*8 

 inches = 250 mm. ; and the optical tube-length at 10' 6 inches 

 = 270 mm. This has caused an increase in the length of the English 

 standard tube, since all good microscopes are made to work with 

 these objectives ; and the addition of a rack and pinion to the l draw- 

 tube ' becomes of great practical value. 



The tube-length of the Continental mechanical tube is 6'3 inches 

 = 160 mm., and the optical tube-length is 7'08 inches =180 mm., and 

 some Continental objectives can only be accurately adjusted on an 

 absurdly short tube of 4| or 5 inches. 



The question has been asked, ' Which is the better of these two 

 differing tube-lengths ? ' So far as the image in the instrument is 

 concerned, there is not much difference. It is of little importance 

 whether the initial magnifying power of an objective be increased 

 by a slightly lower eye-piece used at a longer distance or a slightly 

 deeper (higher) eye-piece at a shorter distance. But it is of practical 

 importance to note that a small difference of tube-length produces a 

 greater effect on adjustment with a short body than with a long one. 

 Critical work is carried on in this country to 2^ mm. adjustment 

 on the long tube ; with a short tube the delicacy would be greater. 

 A difference of 5 mm. on a short tube is equivalent to the difference 

 between a good and a bad objective. When small cones of illumina- 

 tion are used lenses are far less sensitive, but, on the other hand, 

 they are not doing their work. Biologists in a vast majority of cases 

 use a high power insufficiently worked ; thus a J-inch objective with 

 a small cone is used in place of a 1-inch objective, and an oil im- 

 mersion jL-inch objective with small cone is vised to do what a J-inch 

 would have done. The oil r Vinch objective is never fully utilised, 

 and the objects that it will show if properly used are never seen. 

 The principal difference, however, between the long and the short 

 body as affording a datum for their respective values is that when 

 a short body is used by a person having normal accommodation of 

 sight, the stage of the microscope cannot be seen unless the head is 

 removed from the eye-piece, whereas with the long body the eye 

 need not be taken from the eye-piece at all, as the stage can be seen 

 with the unused eye. We are informed by a highly competent 

 German optician that short sight is the most common form of vision 

 amongst German microscopists. This, of course, for Germans so far 

 alters the case, but it does not apply in this country. The diameter 

 of the body tube is also a matter of importance, because when a 

 microscope is used for photomicrography it is essential that it 

 should have a body with a large diameter. 



III. Arrangements for focussing stand next in order of import- 

 ance. Every microscope of the first class is provided with two 

 arrangements for focussing, one a coarse adjustment, acting rapidly, 

 and the other a fine adjustment, which should act with great delicacy 



