THE AMERICAN MONTHLY 



[March, 



When we consider that an apochromatic objective consists of ten 

 separately ground lenses ; that the first lens being more than a hemi- 

 sphere in form is consequently very diflicult to grind ; tliat each lens 

 has to be ground strictly to measure in curvature and thickness ; and 

 that these lenses have all to be put together in a brass mounting so that 

 their distances apart must be adjusted to within half a hundreth of a 

 millimeter, and that when all this is done the objectives are as perfect 

 as it is possible to make them, we may appreciate in some degree 

 the value of mathematical formulae coupled with the finest mathemat- 

 ical skill in perfecting the microscope. 



There is another form of apochromatic objective which has onlv nine 

 lenses and the front is not more than a hemisphere. This will be 

 easier to make, but so far as I know it is not yet on the market. 



Zeiss Sliding Objective Changer. 



The perfection of these new lenses is largely due to the use of the new 

 glass, but I cannot now enter into a description of what is to be seen at 

 the glass-works, nor refer more particularly to the uses of the new ghiss. 



A few words concerning new apparatus may be of interest. The 

 latest form of stand, which will be described in the new illustrated cata- 

 logue soon to be issued, is shown in the frontispiece. It is a compact 

 and convenient stand, well made, carrying an Abbe condenser of the 

 latest construction with iris diaphragm ; price 390 marks. 



An achromatic condenser which is especially recommended for photo- 

 graphing is shown in fig. 1. This condenser is constructed to give 

 a sharp image of the source of light in the plane of the object, for pro- 

 jecting purposes. It has a numerical aperture of i .0. iris diaphragm 

 and centering adjustments. 



