CH. II] LIGHTING AND FOCUSING 43 



ments are for mirror lighting only, remove the subtage condenser 

 if present (see § 90, for condenser). Place a mounted fly's wing 

 under the microscope, put the 16 mm. (^in.) or other low objec- 

 tive in position, also a low ocular. With the coarse adjustment 

 lower the tube of the microscope to within about 1 cm. of the object. 

 Use an opening in the diaphragm about as large as the front lens 

 of the objective; then with the plane mirror try to reflect light up 

 through the diaphragm upon the object. One can tell when the 

 field (§ 57) is illuminated, by looking at the object on the stage, 

 but more satisfactorily by looking into the microscope. It some- 

 times requires considerable manipulation to light the field well. 

 After using the plane side of the mirror turn the concave side into 

 position and light the field with it. As the concave mirror con- 

 denses the light, the field will look brighter with it than with the 

 plane mirror. It is especially desirable to remember that the excel- 

 lence of lighting depends in part on the position of the diaphragm 

 (§ 77)- If the greatest illumination is to be obtained from the con- 

 cave mirror, its ppsition must be such that its focus will be at the 

 level of the object. This distance can be very easily determined by 

 finding the focal point of the mirror in full sunlight. 



§ 80. Use of the Plane and of the Concave Mirror. — The 

 mirror should be freely movable, and have a plane and a concave 

 face. The concaved face is used when a large amount of light is 

 needed, the plane face when a moderate amount is needed or when 

 it is necesssay to have parallel rays or to know the direction of the 

 rays. 



FOCUSING EXPERIMENTS* 



§ 81. Focusing with Low Objectives. — Place a mounted 



\ 82. *Par-Focal Oculars. — By this is meant oculars of different power 

 in which the microscope remains in focus on changing the oculars. 



As originally constructed the microscope had to be focused every time the 

 oculars were changed. Mr. Edward Pennock in seeking to overcome this 

 inconvenience wrote to Professor Abbe for advice in 188 r. After successfully 

 producing oculars of different powers for the Acme microscopes of Jas. W. 

 Queen & Co., according to the directions given by Professor Abbe, Mr. Pen- 

 nock as editor of the Microscopical Bulletin and Science News published in 

 Vol. Ill, 1886, pp. 9-10, the following with Professor Abbe's letter : " Chang- 

 ing Eyepieces without altering focus, etc. Some years ago the writer in 

 looking up certain questions in connection with eyepieces took occasion to 



