34 PHOTOMICROGRAPHY 



this movement. An object such as a diatom should be 

 examined by the highest power likely to be used, and 

 the fine adjustment should be capable of focussing 

 accurately the different planes. There must be no back- 

 lash, and any part that is focussed should remain in 

 focus, even if the instrument receives a slight jar, such 

 as a rap on the table might impart, and no lateral move- 

 ment of the object should be visible when the fine ad- 

 justment is used. It is advantageous to have one of the 

 so-called " Safety " types, by which little or no pressure 

 is applied to the cover-glass if the objective is accidentally 

 brought into contact with the slide, so obviating risk of 

 damage to both preparation and objective. In micro- 

 scopes so fitted the downward movement of the tube 

 depends on its own weight, together with a spring acting 

 against the fine adjustment, and it is well to see that the 

 spring is sufficiently strong to cause the downward 

 motion when the microscope is horizontal. 



Tube Length. English makers used to fit tubes of a 

 fixed length of 10" to their microscopes, but these are 

 now practically obsolete, as the more compact Conti- 

 nental tube of 160 or 170 mm. is so much more con- 

 venient. The modern practice is to make the tubes 

 telescopic, as it is essential, as will be mentioned later 

 when discussing objectives, to be able to vary the tube- 

 length by means of one or two draw-tubes. These draw- 

 tubes should be engraved with a millimetre scale, to 

 indicate the total length in use between objective and 

 eyepiece, and it is very convenient to have a rack-and- 

 pinion movement fitted to one of them. They ought to 

 allow of the extension being varied from about 130 or 140 

 mm. to at least 250 mm., to permit of the use of objectives 

 that have been made for the long tube and for adjustment 

 for cover-glass thickness ; many microscopes, however, 

 only allow of an extension to about 200 mm., and this is 

 generally enough, as objectives are now nearly always 



