126 BELL SYSTEM TECHNICAL JOURNAL 



The objecth'e used with the cardioid condenser is a special apochro- 

 mat 3 mm. 0.85 N.A. glycerine immersion lens which constitutes a 

 homogeneous immersion lens for cover glasses of fused quartz. This 

 type of objective is necessary because the success of the obser\ation 

 is then largely independent of impurities and slight blemishes on the 

 upper surface of the co\'er glass, moreover, the lens confers a greater 

 immunity from the effects of \arying cover-glass thickness and the 

 immersion fluid precludes the entrance of dust which would gradually 

 cloud the image. 



Slit ultra-microscopes are not arranged for photography because 

 in the case of liquids the ijarticles are in a rapid state of motion and 

 the illumination is insufficient. Sinct' in transparent solids the 

 particles are stationary-, the image seen in the slit ultra-microscope 

 may be reproduced by making a lengtlu' exposure. With a small 

 photomicrographic camera develojied h\ the writer the image seen 

 in the slit ultra-microscope for .solids has been reproduced and, b>- 

 instantaneous photograph)-, the mo\ing particles in liquids as seen 

 in the cardioid instrument. P'xcept for pur|)oses of evidence or record, 

 there is Htlle tn lie i^aiiu-d !)>• pluilni^rapliint; with the ultra-microscope. 



i)\RK-('.K(H Ml ll.t.rMtNAIIdN 



The dark-ground illuminator constitutes .iiKitiu-r aid to micro- 

 scopic investigation. This, in realit\-, is a soil nl' ultra-microscope, 

 since the objects are viewed b>- diffrarlcd light much in the .same 

 way as in the cardioid type of eciuipnuiit. This method ol illumina- 

 tion is accomplished by stopping out the axial rays and allowing 

 those of greater aperture to strike the specimen at an angle. The 

 usual form of condenser may be made to yield dark-ground illumina- 

 tion by the simple expedient of inserting a central stop in the path 

 of the light rays just below the sub-stage condenser in a ring provided 

 for such purpose. Better results are attained by use of dark-ground 

 illuminators which are special condensers designed with this object 

 in mind. Dark-ground illumination furnishes valuable means for 

 bringing into view objects which arc smaller than about 1 fx. Kxamples 

 of such objects are furnished by fibers, fine crv-stalline needles, fissures, 

 edges, rods, bacteria, etc. I'nder dark-ground illumination methods, 

 these objects are easily seen and studied, whereas with transmitted 

 light, they can be seen with difficulty unless rendered distinguishable 

 b>' staining. Certain bodies with laminar markings are also suitable 

 subjects for dark-ground studies and in this case the markings are 

 distinguishable more by reason of dissimilarities in refraction than 

 by dilTerences in coloring. 



