EDUCATIONAL VALUE OF PHOTOMICROGRAPHY. 145 



in order to obtain the desired result. Of course with low power objectives 

 one may do this after repeated trials with some accuracy, but the chief 

 difficulty is found in determining whether an indistinct negative is 

 due to improper manipulation, or imperfect resolving power in the 

 objective. For this reason, even for elementary work, such apparatus 

 is not economical and is quite unsatisfactory. 



A consideration of the apochromatic objectives of Professor Abbe 

 and Professor Hastings, which are readily obtained with either the 

 Zeiss or Bausch & Lomb Optical Co.'s instruments, is therefore impor- 

 tant. These objectives having a uniform correction for spherical aber- 

 ration, correct also for three colors, resulting in a better concentration of 

 image rays and greater resolving power ; this improvement in objectives 

 brings the photographer's work nearer perfection, though there is, with 

 high power objectives, an error which thus far remains uncorrected. 

 This defect is balanced, however, by over-corrected eyepieces known 

 as compensating oculars, which are designed for use with these 

 objectives. Thus for photographic work neither an apochromatic objec- 

 tive with the ordinary Huyghenian eyepiece, nor the achromatic objec- 

 tive with compensating eyepiece is satisfactory. 



The bulk of literature on photomicrography advises the use of the 

 objective without any eyepiece; such advice is good with ordinary achro- 

 matic objectives, as the addition of the eyepiece would only introduce 

 more absorbing and reflecting parts without correcting any of the 

 defects in the objective. Where fine work is desired, however, and 

 apochromats are used, the compensating ocular is not only necessary 

 for the highest degree of correction of the system, but is useful in the 

 regulation of magnifying power. 



From the fact that so many different makes of objectives 

 and oculars are in the market, accompanying their respective micro- 

 scopes, the simplest method of rating in photographic work seems to be 

 a statement of the number of diameters which the object is magnified. 

 The magnifying power is most readily and accurately obtained by using 

 a stage micrometer, one millimeter divided into hundredths, and 

 measuring directly the magnification of the image upon the ground 

 glass with an engine-divided steel rule. 



The illumination of the object to be photographed is of so much 

 importance that a few points may be briefly considered. An almost 

 indispensable adjunct of the microscope in this connection is the Abbe 

 sub-stage condenser, which not only condenses the light, but illuminates 

 the object with a cone of light having an angular aperture equal to 

 that of the objective employed. It is arranged to move with rack and 

 pinion, thus providing a means for controlling the illumination to an 

 important extent, Experience shows that the best illumination upon 



VOL. LXI. 10. 



