60 THE AMERICAN MONTHLY [Feb., 



the new formula, and find it to be in all particulars in 

 conformity with the end sought to be attained, a condenser 

 capable of as delicate and accurate focusing as a high- 

 grade objective under any possible use of light, perfectly 

 corrected for both spherical and chromatic aberation, and 

 having a numerical aperture of 1. 30. Workers with the 

 Abbe form of condenser realize the necessity for and in- 

 convenience of constantly changing the focus of the ap 

 paratus, as the method of lighting is changed from cen- 

 tral to oblique ; with the new condenser this is obviated, 

 the focus being constant. An Iris diajjhragm is used with 

 the new condenser, and by the use of an adapter it can 

 be accommodated to any form of sub-stage mounting in 

 use. The name of homogeneous-objective-condenser has 

 been given it, because of the fact that, owing to the per- 

 fect correction of the optical combination, it can be used 

 in the body-tube of the microscope as successfully as any 

 other homogeneous immersion objective. It thus serves 

 a double purpose. The instrument in my possession has 

 been corrected for thin and medium glass slides, and when 

 used as an objective it is better to invert the slide upon 

 the stage. 



In experimenting as to the relative efficiency of the new 

 form and the Abbe condenser, I have selected as a good 

 test a mount of Arnphipletira pellucida, in Smith's me- 

 dium, using a 1-12 Lietz apochromatic homogeneous im- 

 mersion objective. With the use of the Abbe, I only se- 

 cured a resolution of the transverse lines, while with the 

 new form, with the same handling as was given the Abbe» 

 the longitudinal markings were well shown. When using 

 small stops for the purpose of bringing the light to the 

 extreme edge of the objective, the contrast between the 

 work of the two condensers was most strongly marked. 



Various makers have heretofore furnished achromatic 

 condensers of 1. numerical aperture, but this is too 

 small t<) admit of the best results being obtained with 



