APPENDIX. 



Method of measuring an object under the 

 microscope. If while the one 63^6 looks down the tube 

 the other is allowed to remain open, an image of the 

 object will appear projected on the table at the side of 

 the microscope, and it is not difficult to mark off, upon a 

 sheet of paper placed here, the points between which the 

 measurements are to be taken. The preparation is then 

 removed, and a stage micrometer is substituted for it, 

 the parts of the microscope being left in the same condi- 

 tion as before. The stage micrometer is a glass slide on 

 which fine equidistant parallel lines have been ruled with 

 a diamond. The distance between the lines is marked on 

 the. slide ; it is generally either the T <jo tn an( * Wo o tn P arfc 

 of an inch, or the y^th and T ^oth part of a millimetre. 

 The lines are observed with the microscope in the same 

 way as the object, and their image can of course be simi- 

 larly projected upon a sheet of paper and there marked 

 down. The distances between the lines being known, it 

 is easy, by comparison of the two markings, to find out 

 the distance between the opposite points of the object. 



Some microscopes are provided with an eye-piece 

 micrometer (Fig. 33). This is an ordinary ocular witli 

 a flat piece of glass (m) having a scale ruled upon it by 

 a diamond, inserted between the field-glass and eye-glass. 

 The value of the divisions of the scale should lie deter- 

 mined once for all for each objective by observation of a 

 stage micrometer (see Fig. 34), the tube of the micro- 

 scope being fully drawn out, and should be marked on 

 the ocular; and in subsequently using it for measurement 

 all that is necessary is to see how many divisions of the 

 scale the object under examination covers. Thus, sup- 

 posing it had been found by examination of a stage 

 micrometer that with the high power objective and the 



