88 



THE MICROSCOPE 



l/12-inoh 

 abject glass. 



The 1/12-inch (2-mm.) oil-immersion object glass has a maxi- 

 mum field of view of 'OOSB.inch (-2 mm.) and a working distance 

 of '01 inch (1/4 mm.). This is the high-power lens which must be 

 used, if it is necessary to see the finest detail which can be observed 

 with any microscope. It has an aperture of 1-3 N.A. and a 

 theoretical resolving power of 125,000 to 135,000 lines to the 

 inch. It is the object glass that reaches the highest limit yet 

 obtained in microscopic vision, and is a necessary portion of a 

 complete outfit. The apochromatic, being slightly better than 

 the achromatic, is worth the extra cost even if all other lenses 

 are of the achromatic series. If structure of an object is just 

 beyond the Umit of vision of a low-power, a higher power object 

 glass can be used ; but this does not apply to a 1/12-inch, as no 

 higher power will show more ; and if the quality of the highest 

 power lens is such that even shghtly higher power eyepieces can 

 be employed, the scope of the instrument is extended. 



standard 

 size. 



Best 



eyepieces 

 for general 

 work. 



AU eyepieces are made to the diameter of the Royal Micro- 

 scopical Society's No. 1 Standard Drawtube, '917 inch diameter. 

 They are made to drop in loosely, so that they may be changed 

 without any tendency to alter the adjustments of the microscope. 

 They are designated by their focal length, and their magnifying 

 power is given for the distance of distinct vision — 10 inches 

 (250 mm.) — and is engraved on each eyepiece. 



The best eyepieces to use for general work are those of the 

 lowest powers, 42 or 45 mm. and 25 or 30 mm. The eyepoint 

 (T, Fig. 1, page 9) is large with low-power eyepieces, and fine 

 specks of dust on the surface of the eye or in any part of the 

 instrument do not readily show. The higher the power of the 

 eyepiece used, the smaller is the diameter of the eyepoint, and any 

 such minute obstacles to the passage of the light become more 

 apparent. It is, however, of the utmost value to be able to 

 slip in a high-power eyepiece for occasional examinations, in 

 order to increase the power without altering the adjustment of the 



