5S THE AMERICAN MONTHLY [March, 



at least, my own custom with the form having 1.40 N. A., to use it out 

 of focus, except on special occasions ; what others may do I have no means 

 of knowing. The thickness of the slide is, however, to be considered, 

 particularly when using it as an immersion with wide angled glasses. 



When employing low powders with central light or black-ground il- 

 lumination, the concave mirror may be needed to illuminate the entire 

 field ; in other cases the plane mirror is always used, especiallv with the 

 bull's-eye lens. 



Several diaphragms, which are used below the posterior lens, for 

 central, oblique and black-ground illumination, accompany both forms. 

 Those for central light have a central opening, the size of the cone of 

 light and the obliquity of its lateral rays varying with the size of the 

 diaphragm opening employed. For oblique illumination usually two 

 lune-shaped diaphragms are supplied. These are placed in the carrier, 

 one at a time, of course, in anv position that may be needed to produce 

 the eftects desired. For black-ground illumination those with the cen- 

 tral disk supported by radiating arms are used, but to obtain the effect 

 with wide-angled glasses something more is needed than the use of 

 these special disks. A circular diaphragm must also be placed at the 

 back of the objective. 



The diaphragm carrier in the American forms of the condenser is 

 usuallv a sliding plate into whose aperture the various diaphragms are 

 placed, when it is pushed below the lenses imtil a spring catch indicates 

 that it is properly centered to the condenser, but this has nothing to do 

 with the centring of the condenser to the objective. The spring catch 

 is usually a delicate one, and the microscopist is in danger of forcing 

 the carrier beyond the centre. 



The light may be readily modified by the use of the circular diaphrgams. 

 or, if the change is inconvenient, by lowering or raising the condenser. 



For oblique illumination, the lunate disks are used, the larger when 

 the greater portion of the cone of light is to be intercepted, the smaller 

 when more of the rays nearer the centre are desired. With either sized 

 concavitv the condenser will give light of greater obliquity than many 

 objectives will leceive. The object, however, may be obliquely illu- 

 minated with rays from any direction, either by withdrawing the car- 

 rier and inserting the moon-shaped diaphragm in another position, or 

 by rotating the entire condenser, so that the light shall sweep around 

 a circular course. This requires delicate manipulation, an objective 

 of the proper angular aperture to receive light of that obliquity, and 

 very accurate centring of all the parts. It may be done, however, with 

 fine effect in the resolution of lined objects, diatoms for instance, but if 

 the microscopist owns the condenser of i .40 numerical aperture, and 

 this form of oblique light is to be employed, the front hemispherical lens 

 of the condenser should be removed and the remainder of the combina- 

 tion focused on the object without any diaphragm. Then insert the lu- 

 nate disk, and, if all is well, a glance down the body tube, without the 

 ocular, will show a small double-convex spot of light near one border 

 of the back lens of the objective, with the diffraction spectra also, if 

 they are specially looked for, particularly if Plciu-osigiua angulatum 

 be the object on the stage. With the condenser of 1.40 N. A., the 

 front lens being removed, and the lunate diaphragm in the carrier, Mr. 

 Gundlach's dry one-fifth inch objective, 135°, resolves balsam mounted 



