40 PEACTICAL PHOTO-MICEOGEAPHY 



were first designed by Professor Abbe, it was found con- 

 venient in the low powers to introduce a chromatic error 

 which the ocular could correct. Consequently, if an apochro- 

 matic objective is used with an ordinary ocular it does not 

 appear to be perfectly corrected, the actual condition being 

 that the different coloured images are of varying dimensions. 

 Most makers now arrange that for visual work their series of 

 oculars, whether compensating or otherwise, are so made that 

 when dropped into the body-tube of the microscope they 

 are in their correct position that is, no alteration of the 

 mechanical tube-length is necessary, as shown in Fig. 11. It 

 is for this reason that the compensating oculars of Zeiss are 

 made in both the Huyghenian and Kamsden types ; for it is only 

 by this method that the lower focal planes of the whole series 

 of oculars can be brought into the same position when inserted 

 in the draw-tube of the microscope. Also, when a change 

 of eye-piece becomes necessary very little alteration of the fine- 

 adjustment is required to bring each objective into exact focus. 



The ' tube-length,' or the distance between the ocular and 

 objective, in part determines the magnifying power of the 

 system. Two lengths are in general use. Most English 

 microscopes are fitted for what is known as the * long ' 

 tube, with a distance of 10 inches or 250 mm. between 

 the upper focal point of the objective and the lower 

 focal point of the eye-piece. In the Continental microscopes 

 this tube-length is 160 or 170 mm. (usually the former), and 

 the tube-length should not be varied from the length stated by 

 the maker, unless for the purpose of securing better correction 

 for variations in the thickness of the cover -glass. Further 

 reference to this will be made, when the methods of using the 

 various optical parts in conjunction are described, in Chap. VII, 

 p. 154. In the case of .compensating oculars these are 

 distinguished by numbers, and the number represents the 

 number of times that the eye-piece magnifies the image formed 

 by the objective when it is used with its standard tube-length. 



For photography, projection-oculars will be required 

 when apochromatic objectives are used ; with achromatic 

 objectives they are also advisable under certain conditions. 

 If an ocular is used at all, they are in fact the only reliable ones. 

 In these the same principle is adopted as in the case of the 



