300 The;Microscope. 



then attached in the well known way to a wooden slip with a 

 central hole of sufficient size. This mode of mounting made it 

 possible for me to use a homogeneous immersion objective as a 

 sub-stage condenser, as I did. 



The objective in use on the tube of the microscope was 

 a iV homogeneous immersion, 1.30 N. A., the condenser was a 

 fine i homogeneous immersion objective, 1.29 N. A., attached to 

 the lower thin glass by homogeneous immersion fluid. The 

 image of the narrow edge of the lamp flame was carefully 

 focussed on the object by the condenser. 



I took direct light from the lamp and was as careful to place 

 it at the proper distance from the condenser — ten inches — as 

 I was to adjust the tube length of my microscope. 



The condenser was very carefully centred, and gave a cone of 

 light as free from spherical and chromatic aberration as I think 

 can be obtained. I used the entire cone, and also used small 

 stops for central and oblique light. Small dark stops to cover 

 the central portion of the condenser were used. 



All modes of use gave an image in the eye-piece far in advance 

 of that obtained by the Abbe condenser — and I have a good 

 one — as the enormous spherical aberration of the Abbe instru- 

 ment renders it impossible to obtain an exact focus of the source 

 of light upon the object. 



I am well aware that a condenser of the kind I used will not 

 come into popular use at the present price of homogeneous ob- 

 jectives, but the object of the present paper is to call attention 

 to the importance of having sub-stage condensers properly 

 corrected for exact focus, and to the careful adjustment of the 

 condenser when put upon the sub-stage. I have seen owners of 

 fine objectives put their condenser, it seemed to me, almost 

 anywhere below the stage and be perfectly satisfied with the 

 result. Of course their objectives were not doing their best 

 work with this treatment. 



Some of the world's famous opticians are now furnishing 

 achromatic condensers of from 1.0 N. A. to 1.40 N. A., with 

 corrections almost as perfect as in their superb objectives, and 

 of proper focal power. Of course I would not advise the use 

 of a combination for condenser of as high a power as a ^ objec- 

 tive ; a "1^ to ^ inch would be much better. 



