The Microscope. 



291 



for the long or English "tube-length," and 160 mm. for the shorter 

 Continental "tube-length.'" Furthermore, the same parts of the micro- 

 scope should be included in the tube-length, and the parts included 

 should be readily detei'minable by the youngest student. The parts 

 inclnded by six of the opticians named above, viz: from the top of 

 the ^ tube (6) where the ocular is inserted, to the lower end (a?) 

 where the objective is screwed in, answers this requirement of sim- 

 plicity. Without urging this as the best possible selection, it will 

 readily be seen that this " tube-length " may be easily measured 

 where the ocular and objective are not in position, and that makers 

 of stands who do not also make objectives could easily make the 

 tubes of their microscopes of exactly the right length for the 

 objectives of all objective makers. While it is true 

 that the objectives of various makers are in movint- 

 ings of different lengths, and, therefore, other 

 things being equal, tend to increase or diminish 

 the actual or " optical tube-length,'" and thus to 

 vary the magnification of the microscope, if each 

 maker would choose the length designated above 

 ( b-d ) for which to correct his objectives in their 

 mountings, then no matter how long or short that 

 mounting might be, the microscopist would be able 

 to measure off the right length on the tube of his 

 microscope, for which the objective was cori'ected, 

 and having this length once determined, it would 

 not need to be changed when an objective of 

 different length of setting was used. 



Furthermore, the convenience of the micro- 

 scopist and uniformity in " tube-length " would 

 be both subserved if the eye-pieces or oculars were made "par- 

 focal,'"' * " that is the settings be so adjusted that the lower focal 

 points of all the eye-pieces shall be at the same level when in posi- 

 tion in the tube of the microscope," f then no refocusing of the 

 microscope would be necessary upon changing oculars. If also 

 the level of the " lower focal points " of the different oculars were 

 made to fall at the level of the top of the body tube of the micro- 

 scope, one end of the so-called "optical tube-length" would be 

 always determinable, and correspond with one end, that is the upper 

 end, of the tube of the microscope. 



* This convenient name was proposed by the editor of the Microscopical Bulletin in 

 vol. iii., (1886), p. 31. See also p. 9, same vol. 



+ See page 8 of the catalogue sent out by Zeiss, with his apochromatic objectives and 

 compensation oculars. Also, Joiu: Roy. Micr. Soc, 1886, p. 853. 



