82 



THE MICROSCOPE 



Sizes of 

 field. 



of view with various object glasses at the standard tube length, 

 and the working distance of each lens. 



standard 

 screw of 

 object 

 glasses. 



Variety of 

 object 

 glasses 

 required. 



1 Those marked ^ are apochromatic. The other object glasses are 

 achromatic. 



All object glasses are now made with a screw standardised 

 by the Royal Microscopical Society ; its specification is as follows : 



Thread, Whitworth Screw, 36 to the inch, length ^125 inch. 



Plain fitting on object glass above screw, •! inch long, ^759 

 inch diameter. 



Diameter of thread of object glass at top of thread, -7952 to 

 •7982 inch. 



Diameter of thread of object glass at bottom of thread, -7596 

 to -7626 inch. 



Diameter of thread of nosepiece at top of thread, '7644: to 

 •7674 inch. 



Diameter of thread of nosepiece at bottom of thread, -8 to 

 •803 inch. 



The stem of all the Beck object glasses are of a standard 

 diameter, '65 inch. 



The series of object glasses mentioned in this book is sufficiently 

 large to cover all the requirements of microscopy, with the possible 

 exception of a very low power for examining unusually large 

 objects. The highest power is a l/12th inch. It has sufficient 

 magnifying power to show all the detail that the maximum aperture 

 will resolve; and although object glasses of higher power can be 

 made, they cannot be made with this maximum aperture because 

 the lenses must be much smaller, and as a fixed distance must be 

 allowed for a cover glass they cannot collect as large an angle. 



Object glasses of imtermediate sizes have also been made 

 from time to time, but the magnifying power given by an inter- 

 mediate size can be so readily obtained by a change in the eye- 

 piece that they are of little advantage. The manufacture of 



