132 Transactions of the Society. 



piece has a collar or ring fitted to it which raises its front focus to 

 about the level of the top of the tube. 



In column F the result of such a mechanical tube length upon 

 the value of A is clearly seen. Column H shows the percentage of 

 loss or gain in power, owing to the difference between the actual optical 

 tube length A and a theoretical one of 10 in. It will be seen that 

 there is an error of 8 per cent, with a 1 in. objective ; but with powers 

 higher than that it quickly vanishes. The error with very old ob- 

 jectives is necessarily greater, because they have shorter mounts. 



In a Microscope with a short tube A = 6 in., consequently if we 

 want p to equal m m' we must adopt an optical fiction, and assume for 

 the same objective a different initial power when it is used upon a 

 short tube than when upon a long tube. Thus the initial power 



of a £ in. is _— ■ = 20 ; but if we wish to use this lens upon a short 

 tube we must consider it as - or 12 ; moreover, we must treat the 



2 



eye-piece differently, and leave it the same value as it had before ; 

 then, when we have assigned these values to m and m', the power 

 p of the short tube Microscope will be represented by their product. 



The firm of Messrs. Zeiss treat the subject differently, and cata- 

 logue the power of their objectives as calculated for the long tube, 

 and give a fictitious value to the magnification of their eye-pieces ; 

 thus, for example, the same eye-piece is catalogued as an 8-power 

 for the short and as a 12-power for the long tube. 



In 1860 the Wenham binocular was invented, and the effect of its 

 introduction was to lengthen the tubes in order to avoid too steep an 

 angle of convergence for the eyes ; to this may be traced the origin of 

 those Microscopes one occasionally sees, with enormously long bodies. 

 Powell's tube length was hardly affected by the Wenham binocular, 

 because a separate monocular body was provided. 



If, in conclusion, these few remarks should induce any maker to 

 lengthen the mount of his low-power objectives and shorten those of 

 high power, and at the same time make his Huyghenian eye-pieces so 

 fit the body tube that the front foci will all lie at the top of the tube, 

 they will not have been written in vain. 



Note. — If any one wishes to obtain hypercritically approximate results, when 

 dealing with very low-power objectives by the formula in column C, be may apply a 



100 

 correction by subtracting -, — r~T\5 from the values given there. But this correc- 

 tion should be applied only to very low-power objectives, where m is less than 50. 

 This correction, like the principal formula, may be found without calculation in 

 Barlow's Tables (Spon), a book indispensable to all microscopists interested in 

 optical science. The Microscope lens of lowest power is nominally a G in. (actually 

 a 4 in.) ; but when the correction is applied, the formula is applicable to photographic 

 lenses up to 15 in. of focus, a distance of 100 in. between the object and image being 

 maintained. 



