4 o LIGHTING AND FOCUSING [ CH. II 



millimeters or % inch from the object when the microscope is in focus, but 

 considerably less than that distance. If there were no other reason than the 

 limited working distance of high objectives, it would be necessary to use a 

 very thin cover-glass over the object. (See \ 27, 33.) If too thick covers are 

 used it may be impossible to get an objective near enough an object to get it 

 in focus. For objects that admit of examination with high powers it is always 

 better to use thin covers. 



I 70. Free Working Distance — In the microscope catalog of Zeiss there 

 is givena 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 immediately 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 unadjus- 

 table objectives are corrected. If, for example, the thickness of cover for 

 which an objective is corrected is fifc 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 thick- 

 ness, I 33). 



LIGHTING WITH DAYLIGHT 



\ 71. Unmodified sunlight should not be employed except in special 

 cases. North light is best and most uniform. When the sky is covered with 

 white clouds the light is most favorable. To avoid the shadows produced by 

 the hands in manipulating the mirror, etc. , it is better to face the light; but 

 to protect the eyes and to shade the stage of the microscope some kind of 

 screen should be used. The one figured in (Fig. 66) is cheap and efficient. 

 If one dislikes to face the window or lamp it is better to sit so that the light 

 will come from the left as in reading. 



It is of the greatest importance and advantage for one who is to use the 

 microscope for serious work that he should comprehend and appreciate thor- 

 oughly the various methods of illumination, and the special appearances due 

 to different kinds of illumination. 



Depending on whether the light illuminating an object traverses the object 

 or is reflected upon it, and also whether the object is symmetrically lighted, 

 or lighted more on one side than the other, light used in microscopy is des- 

 ignated as reflected and transmitted, axial and oblique. 



\ 72. Reflected, Incident or Direct Light. — By this is meant light reflected 

 upon the object in some way and then irregularly reflected from the object to 

 the microscope. By this kind of light objects are ordinarily seen by the 

 unaided eye, and the objects are mostly opaque. In Vertebrate Histology, 

 reflected light is but little used ; but in the study of opaque objects, like 

 whole insects, etc., it is used a great deal. For low powers, ordinary daylight 

 that naturally falls upon the object, or is reflected or condensed upon it with a 

 mirror or condensing lens, answers very well. For high powers and for 



