58 THE USE OF THE MICROSCOPE 



eyepieces 12.5 (7), on the Bitumi binocular attachment. 

 For the apochromatic microscope there were employed 

 objectives 10, 20, 60 oil of aperture 1.3 and 60 oil of aperture 

 1.4; all with paired eyepieces 20. This is for objects in 

 immersion oil (or balsam). For general objects, apochro- 

 matic objectives 10, 20, 70 water immersion, and 90 oil of 

 1.3 aperture can be used with paired eyepieces 15. Thus 

 little or no changing of eyepieces may be needed in 

 ordinary high-power work. 



Microscope Body. — In the writer's opinion, the micro- 

 scope should be capable of being readily screwed down to 

 the table or to a board. The instrument should be inclin- 

 able about 40 degrees from the vertical, and should rest 

 there against a stop. If there is no stop, one can usually 

 be improvised. 



Summary. — The monocular microscope, with a ground- 

 glass screen for the unused eye, is suited for work which 

 only occupies an hour or two at intervals. For regular 

 continuous work, day after day, for years, the binocular is, 

 in the writer's experience, almost necessary. For routine 

 work, the drawtube can well be omitted, as being a frequent 

 source of error. By the use of the drawtube, the skilled 

 microscopist can partially correct his low objectives for 

 uncovered objects, and his medium or high dry objectives 

 for slightly different thicknesses of cover-glass, etc. (Large 

 variations in cover-glass thickness should not occur.) 

 Hence, on the monocular with graduated drawtube, the 

 40 dry objective, of 0.85 aperture, can be fairly satisfac- 

 torily used without a correction collar; which is not the case 

 on the binocular. However, the more correct the cover 

 thickness, the less the drawtube need extend or contract. 

 It is easier, and perhaps often better, to change objectives 

 than eyepieces. 



Practical Points 



1. It has been found that with constant use of the monocular, 

 the employment of a translucent screen over the unoccupied eye 

 enables work to bo carried on without overtiring the observer. 



