io MICROSCOPE AND ACCESSORIES. 



with the microscope. While they are specially adapted for use with apochromatic 

 objectives, they may also be used with ordinary achromatic objectives of large 

 numerical aperture. 



\ 25. Micrometer Ocular. — This is an ocular connected with an ocular microme- 

 ter. The micrometer may be removable, or it may be permanently in connection 

 with the ocular, and arranged with a spring and screw, by which it may be moved 

 back and forth across the field. (See Ch. Ill, under Micrometry). 



\ 26. Spectral or Spectroscopic Ocular. — (See Micro-Spectroscope, Ch. IV). 



DESIGNATION OF OCULARS. 



\ 27. Equivalent Focus. — As with objectives some opticians designate the ocu- 

 lars by their equivalent focus (\ 7). With this method the power of the ocular 

 varies inversely with the focal length, i. e., the less the equivalent focus the greater 

 the power, and the greater the focal length the lower the power. 



\ 28. Numbering and Lettering. — Oculars like objectives may be numbered or 

 lettered arbitrarily. When so designated, the smaller the number, or the earlier 

 the letter in the alphabet, the lower the power of the ocular. 



\ 29. Magnification or Combined Magnification and Equivalent Focus. — The 

 compensating oculars are marked both with their equivalent focus and the amount 

 they magnify the real image. Thus, an occular marked xzj., 45 mm., indicates 

 that the equivalent focus is 45 milimeters, and that the real image of the objective 

 is multiplied four-fold by the ocular. 



The projection oculars are designated siuiply by the amount they multiply the 

 real image of the objective. Thus for the short or 160 mm. tube-length they are, 

 x 2, x 4 ; and for the long, or 250 mm. tube, they are x 3 and x 6. Tbat is, the 

 final image on the screen or the ground glass of the photographic camera will be 

 2, 3, 4, or 6 times greater than it would be if no ocular were used. 



COMPOUND MICROSCOPE. 



EXPERIMENTS 



§30. Putting an Objective in Position and Removing it. — Ele- 

 vate the body of the microscope by means of the coarse adjustment 

 (Fig. 10), so that there may be plenty of room between its lower end 

 and the stage. Grasp the objective lightly near its lower end with two 

 fingers of the left hand, and hold it against the nut in the lower end of 

 the body (Fig. 10). With two fingers of the right hand take hold of 

 milled ring near the back or upper end of the objective, and screw it 

 into the body of the microscope. Reverse this operation for removing 

 the objective. By following this method the danger of dropping the 

 objective will be avoided. 



§ 31. Putting an Ocular in Position and Removing it. — Elevate 

 the body of the microscope with the coarse adjustment (Fig. 10), so that 

 the objective will be 2 cm. or more from the object — grasp the ocular 

 by the milled ring next the eye-lens (Fig. 5), and the coarse adjust- 

 ment or the tube of the microscope and gently force the ocular into 



