1 Those marked ^ are apochromatic. 

 achromatic. 



The other object glasses are 



AH object glasses are now made with a screw standardised 

 standard by the Royal Mioroscopical Society ; its specification is as foUows : 



screw of _^_^ 



object Thread, Whitworth Screw, 36 to the inch, length -125 inch. 



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

 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 aU 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 



Variety of 

 object 



required. 



