USE AND MANIPULATION OF MICKOSCOPE 151 



which the objective has been designed. With low powers some 

 small variation from the correct tube-length is permissible, 

 but this does not apply to a medium or high power. It is 

 unfortunate that makers have not adopted a uniform standard 

 tube-length. Some compute their objectives to work with a 

 length of 160 or 170 millimetres, usually referred to as the 

 short-tube ; while others use what is known as the English tube 

 that is, a length of 220 millimetres. As explained later, some 

 variation of these tube-lengths may, under certain conditions, be 

 necessary ; but for low- or medium-power objectives, or for oil- 

 immersions, it will be advisable not to vary the tube-length 

 from that stated by the maker to be the correct one. The 

 majority of objectives are now corrected either for 160- or 170- 

 millimetre tubes ; and all microscopes have sufficient length of 

 tube to accommodate themselves to the required condition. 

 It is important to remember that in adjusting this tube-length 

 due allowance must be made for any objective-changer that may 

 be on the microscope. This again varies considerably according 

 to the custom of the maker. The Zeiss sliding changers add 22 

 millimetres to the tube-length, and in general it may be taken 

 that no changer adds less than this. The only satisfactory 

 way is to ascertain when purchasing any such appliance the 

 actual allowance that it entails. 



In practice it is desirable to prevent as far as possible any 

 dust or dirt from reaching the objectives, and when one is 

 removed from its case, both the box portion and the lid of the 

 case should be inverted on the table to prevent the entry of 

 dust. A well-washed piece of linen or silk should be kept 

 specially for the purpose of cleaning an objective. It is advisable 

 to keep a small piece in a stoppered bottle or some such closed 

 receptacle, and only to take it out as required, replacing it 

 immediately after use. To clean immersion objectives a very 

 slight moistening of the rag with zylol or benzole may be 

 recommended. Any excess of either of these solvents is to 

 be carefully guarded against, as it may injure the objective if 

 it reaches the inside of the lens mount. Possibly a better 

 material for cleaning purposes in general, however, is Japanese 

 silk paper ; a small piece of this may be torn off when required, 

 and can then be thrown away after use, so that no dirt can 

 accumulate on it. 



