CH. /] MICROSCOPE AND ACCESSORIES 17 



\ 32. Tube-length. "In the construction of microscopic objectives, the 

 corrections must be made for the formation of the image at a definite distance, 

 or in other words the tube of the microscope on which the objective is to be 

 used must have a definite length. Consequently the microscopist must know 

 and use this distance or ' microscopical tube-length ' to obtain the best results 

 in using any objective in practical work." Unfortunately different opticians 

 have selected different tube-lengths and also different points between which 

 the distance is measured, so that one must know what is meant by the tube- 

 length of each optician whose objectives are used. See table, \ 34. 



\ 33. The Thickness of Cover-glass used on an object (See Ch. VII, on 

 mounting), except with homogeneous immersion objectives, has a marked 

 effect on the light passing from the object (Fig. 64). To compensate for this 

 the position of the systems composing the objective are closer together than 

 they would be if the object were uncovered. Consequently in non-adjustable 

 objectives some standard thickness of cover-glass is chosen by each optician 

 and the position of the systems arranged accordingly. With such an objective 

 the image of an uncovered object would be less distinct than a covered one, 

 and the same result would follow the use of a cover-glass much too thick. 



\ 34. In the following tables tube-length b-d of the diagram greatly pre- 

 ponderates, and a large majority of unadjustable objectives are corrected for a 

 thickness of cover-glass falling between fifteen and twenty hundredths of a 

 millimeter (0.15-0.20 mm.). 



* The information contained in the tables on the following page was very 

 kindly furnished by the opticians named, or obtained by consulting catalogs. 

 In most of the later catalogs the information is definite, and many makers 

 now not only put their names and the equivalent focal length on their objec- 

 tives, but they add the numerical aperture (\ 36) and the tube-length for 

 which the objective is corrected. This is in accordance with the recommenda- 

 tions of the author in the original paper on "tube-length," (Proc. Amer. Soc. 

 Micr., Vol. IX., p. 168, also by Edward Bausch, Vol. XII., p. 43). If the 

 table in this edition is compared with the original table or with that in the 

 previous editions of this book some differences will be noted, the changes 

 being in the direction of uniformity and in general in the direction recom- 

 mended by the writer and Mr. Edward Bausch and the committee of the Amer- 

 ican Microscopical Society. The recommendations of the committee were 

 published in the Proceedings, Vol. XII. p. 250. 



