164 PRACTICAL PHOTO-MICROGRAPHY. 



short of loss of definition and resolution. Definition and 

 resolution as technical terms must not be confounded. Reso- 

 lution consists in visibly separating close markings, definition 

 consists in imaging distinctly small compact objects. 



In order to obtain the full benefit of the series of apochro- 

 matic objectives made by Zeiss it is necessary to use in 

 combination with these objectives the "compensating eye- 

 pieces" made to go with them. In order to obviate the 

 necessity for an ocular being specially made to suit each 

 objective, Abbe and Zeiss have been bold enough to deliberately 

 introduce in certain objectives certain aberrations which are 

 corrected by the oculars. 



For the projection of a real image, such as in photo- 

 micrography we require, free from aberrations, and visually 

 and chemically correct, Abbe has designed and Zeiss makes a 

 series of u projection oculars." These are the oculars to which 

 we have referred as being the best, if not the only good, 

 oculars for photo-micrography, 



In both the compensating and projection eye-pieces Zeiss 

 follows the commendable system of marking the eye-pieces, not 

 by arbitrary and meaningless letters, as " A," " B," or " C," 

 but with a number indicating the amount to which the object- 

 ive image is magnified by the ocular ; but it is to be noted in 

 calculating the magnification of an image produced by these 

 oculars, that the figure on the ocular is accurate only for the 

 precise tube length for which the ocular is designed. The 

 No. 4 ocular is intended for the continental tube of 160 milli- 

 meters (about 7 inches); and at 160 mm. behind the posterior 

 conjugate focus of the objective, the objective-image is magnified 

 just four times by the No. 4 ocular ; but if we are caused to 

 alter the tube-length in order to obtain better " correction for 

 the cover glass," the calculation no longer can be taken as 

 accurate, as far as regards our total magnification. 



The magnification given by a projection ocular and objective 

 combined on a screen at a known distance from the ocular 

 may, with convenience, but only in one case with absolute 

 accuracy, be calculated by dividing the distance in mm. from 

 ocular to screen, multiplied by the number on the ocular, by 



