and Laboratory Methods. 2593 



Industrial Microscopy. 

 II. 



APPARATUS. 



The first to be considered under this head is the microscope itself. The 

 compound microscope is an instrument consisting of two combinations of lenses 

 supported in place by a tube of metal. For the adjustment of the system with 

 respect to the object the tube is controlled by some form of device for coarse 

 movement and, usually, some form of micrometer movement by which the whole 

 system is accurately moved through short distances. 



The combination of lenses nearest the object is known as the objective, while 

 the set nearest the eye of the observer is called the ocular or eyepiece. The 

 objective projects a real image of the object at a distance back of the lens. The 

 exact location of this image depends upon the relationship between the focal 

 length of the objective and its distance away from the objective. When in 

 proper relationship to the object a real image of it is brought to a focus in the 

 plane of the diaphragm of the ocular by means of the objective and first lens of 

 the ocular combination. The upper lens of the eyepiece treats this image as a 

 real object, magnifying it again according to its power. 



There are at present so many types of instruments upon the market that the 

 final choice must be made by the worker himself in view of the work to be 

 accomplished and the outlay he has at his command for the purpose. A few 

 general suggestions may perhaps not be out of place at this time, however. 

 Through the courtesy of the Bausch & Lomb Co., the writer is enabled to show 

 in Fig. 1 a cut of one of their make of instruments embodying many of the 

 features mentioned further on. For many lines of work a simpler type would be 

 found adequate. In purchasing an instrument for general work a stand should 

 be secured allowing the use of substage condenser, polarizing apparatus together 

 with the selenite disk, and furnished with ready means of controlling, by means 

 of a suitable substage diaphragm, the cone of light entering the objective. 



The lenses on an instrument should be so chosen as to be capable of giving 

 a range of magnifications from 60 to 300 diameters for ordinary work, though a 

 lower power (about 30) is often useful as a searcher and in use for measuring 

 fibers or other long bodies. On the other hand, for some small objects such as 

 some of the starches a power of 500 is useful. Such combinations can of course 

 be secured by consulting the catalogues of the makers of microscopes. 



But magnification is by no means the only or even the most important requi- 

 site in a microscope. The power of resolution and definition are without doubt 

 more important. Many times a lens of comparatively low power has been found 

 more serviceable than one of higher power, for the reason that it had the better 

 defining power. Flatness of field, numerical aperture and achromatism are 

 important factors to be considered since they enter into determining the defining 

 and working value of the lens. 



Many i/i teres ting features may be seen through almost any lens, but when it 



