PHOTOMICROGRAPHY AND TECHNICAL MICROSCOPY 771 



of better materials, possesses better meclianical and optical features, can be operated 

 more easily, and is readily and conveniently altered to suit a wide diversity in work. 

 Research laboratories are having little difficulty with the present inverted-type micro- 

 scope whereas older apparatus was a constant source of trouble, largely because the 

 assembly was flexible on an optical bench and was not easily aligned and coordinated. 

 The older equipment also had mechanical weaknesses which tended to introduce 

 optical misalignment often difficult to recognize and rectify. 



By photomicrography is meant the adaptation of photography to microscopy or 

 the art of photographing a magnified image. The scope of the art embraces the 

 reproduction of images ranging from actual size to magnifications of several thousand 

 times. Low-power photomicrography may be considered as treating with magnitudes 

 from about 1 to 30 diameters, medium-power work with magnifications from about 

 30 to 500 diameters, and high-power photomicrography generally includes all magni- 

 fications in excess of about 500 diameters. The apparatus used in each case is differ- 

 ent, and the preparation of the material and its treatment also differ. For low-power 

 work the microscope is often dispensed with entirely, the lens being secured directly 

 to the camera; in other cases the microscope serves only as a support for a specialized 

 camera lens. In the treatment of most transparent mounts an illuminating device 

 termed a "substage condenser" is necessary, the microscope then forms a very neces- 

 sary adjunct to low-power photomicrography. 



Medium-power photomicrography always requires a microscope, and because 

 rigidity in mounting and accuracy in adjustment are necessary, an optical bench is 

 desirable on which the microscope and a suitable illuminating train are mounted. 



For high-power photomicrography great attention is given to the mechanical and 

 the optical design of the apparatus. It is extremely important to prevent shocks, 

 vibrations, and extraneous disturbances from reaching the optical bench or camera. 

 It is also important that the apparatus and the camera vibrate or move as a unit and 

 that vibrations are not introduced, through use of the apparatus, which will result in a 

 slight change in focus of the microscope objective. The operations of removing a 

 focusing screen and substituting a plateholder can introduce sufficient shock to throw 

 the image completely out of focus unless the mechanical design has been carefully 

 worked out. For high-power photomicrography only the best optical systems should 

 be employed. A skilled technician may produce remarkable medium-power results 

 with quite ordinary apparatus but in high-power photomicrography nothing can 

 make up for the actual shortcomings of an objective. 



Optical System of Microscopes. — The optical system of the compound microscope 

 is shown diagrammatically in Fig. 1. In the diagram three parallel pencils of light are 

 shown reflected upward into the condenser which illuminates a transparent object 

 placed in position on the microscope stage. As shown, the objective would form an 

 inverted real image of the object Oi at O2 but the rays are intercepted by the lower 

 lens of the eyepiece before the real image is formed. The lower eyepiece lens in com- 

 bination with the upper eyepiece lens forms a magnified virtual image O4 of the real 

 image O2. There are two magnifications of the object, and the resulting final magni- 

 fication is the product of the magnifying powers of the objective and the eyepiece. 



It should be noted that the objective produces an enlarged image of the object and 

 that the eyepiece further magnifies this image; from this it is evident that if detail 

 is lacking or if the image is not a good likeness of the object, the eyepiece will not make 

 up for the shortcomings of the objective. The objective then becomes the most 

 important part of the microscope. No one objective will serve all purposes because of 

 the limited range throughout which each particular objective is most useful; hence it 

 is desirable to have a representative group of objectives available so that an objective 

 may be selected to suit the specimen and the requirements of the work, 



