40 LIGHTING AND FOCUSING [CH. II 



the objective is in focus push the wedge under the objective on the 

 slide until it touches the objective. Mark the place of contact with a 

 pencil and then measure the thickness of the wedge with a rule 

 opposite the point of contact. This thickness will represent very 

 closely the working distance. For measuring the thickness of the 

 wedge at the point of contact for the high objective use a steel scale 

 ruled in ^ths mm. and the tripod to see the divisions. Or one may 

 use a cover-glass measure (Ch. VIII) for determining the thickness of 

 the wedge. 



For the higher powers if one has a microscope in which the fine ad- 

 justment is graduated, the working distance may be readily determined 

 when the thickness of the cover-glass over the specimen is known, as 

 follows : Get the object in focus, lower the tube of the microscope, un- 

 til the front of the objective just touches the cover-glass. Note the 

 position of the micrometer screw and slowly focus up with the fine 

 adjustment until the object is in focus. The distance the objective was 

 raised plus the thickness of the cover-glass represents the working dis- 

 tance. For example, a 3 mm. objective after being brought in contact 

 with the cover- glass was raised by the fine adjustment a distance repre- 

 sented by 1 6 of the divisions on the head of the micrometer screw. 

 Each division represented .01 mm., consequently the objective was 

 raised . 16 mm. As the cover-glass on the specimen used was . 15 mm. 

 the total working distance is .16 +.15 =-31 mm. 



| 76a. Free Working Distance. In the microscope catalog of Zeiss there is 

 given a table of the size of the field and also of the "free working-distance." This 

 free working-distance is the space between the lower end of the objective and the 

 cover glass of T y^mm. thickness, when the objective is in focus on an object imme- 

 diately under the cover. This is exceedingly practical information for a possessor 

 of a microscope, and it is hoped that the other opticians will adopt the suggestion. 

 Naturally, however, the free working-distance for each optician should be reckoned 

 from the top of the cover for which his unadjustable objectives are corrected. If, 

 for example, the thickness of cover for which an objective is corrected is -^ mm. 

 then the free working-distance should be that between the top of this and the 

 objective when the objective is in focus on an object under the cover. (See the 

 table of cover-glass thickness, p. 14). 



CENTRAL AND OBLIQUE LIGHT WITH A MIRROR 



77. Axial or Central Light ( 65). Remove the condenser 

 or any diaphragm from the substage, then place a preparation contain- 

 ing minute air bubbles under the microscope. The preparation may 

 be easily made by beating a drop of mucilage on a slide and covering 



