274 ACCESSORY APPARATUS 



microscope. This consists of a strong upright, fitted into a massive 

 tripod 01- circnLir foot. The foot in either case only rests on three 

 I mints ; the upright is capable of telescopic extension by a clamping 

 tube ; a short tube which takes the eye-piece is fixed to this upright 

 liv .1 compass joint. 



To use it. the object to be measured is placed in position, and 

 the microscope inclined in the usual way. The ordinary eye-piece 

 is removed, and the separate stand with the micrometer in its place 

 is put in front of the microscope, the extension tube being raised or 

 lowered until the tube at the top of it, carrying the micrometer, is 

 made continuous with the tube of the microscope, as seen in the 

 drawing. It is well to leave from ^th to -j^ths of an inch of space 

 between the body-tube and the micrometer tube. It will be now 

 needful to employ corrections to compensate for the increased length 

 of tube. If the objective be provided with a ' correction collar ' the 

 adjustment must be re-corrected ; hut if it is not so provided the tube 

 of the microscope must be shortened exactly as much as the tube 

 carrying the micrometer will have lengthened it. 



By this arrangement it will be found that manipulation can be 

 effected without the vibration of the microscopical image which is in- 

 evitably the result of the revolving of the micrometer screw head when 

 the micrometer eye-piece is placed, as it usually has been, in the body- 

 tube of the microscope. The consequence is that much more minute 

 spaces can be measured, and with much greater accuracy. Mr. 

 Nelson has repeatedly spanned the -j-gVoth f an " lcn by means of 

 a stage micrometer in the focus of the objective ; this was replaced 

 by a mounted specimen of A nip1iii>1<>nra pellnclda, and he has counted 

 ninety-six lines in the t Voth "^ ;IU inch by making the movable wire 

 pass successively over them until the fixed wire was reached. By 

 similar means the Editor has measured single objects less than the 

 iWoiroth f an inch. 



It will have been premised by the careful reader that the stage 

 micrometer must be used in every set of measurements ; at least we 

 would strictly emphasise this as the only accurate and scientific 

 method. It has been advised that a record of comparisons with the 

 various lenses in the possession of the microscopist should be made 

 once for all. We decidedly deprecate this method, unless it be in 

 such utterly valueless work, as is sometimes done, where lenses are 

 (incorrect ed and accuracy of tube-length forgotten or ignored. The 

 correction of an objective and the tube-length ought to vary with 

 every object, and therefore a comparison of the stage-micrometer 

 and the screw-micrometer should lie made with every set of measure- 

 ments. 



Moreover, the majority of -ta^e micrometers exhibit very con- 

 siderable discrepancies in the several intervals between the line*: 

 it is \\ell in the interests of ace u racy to take t he screw value of each 

 under a high power, liud the value of the average, and then note the 

 particular space or spaces that may lie in agreement with the average 

 and always use it. An illustration will make this clear. 



provides a stage micrometer of I mm. divided into '1 and 



