36 Modern Microscopy 



Steinheil. Zeiss also makes most excellent dissecting 

 microscopes and well corrected magnifiers. Dissecting 

 microscopes of various patterns may also be found in 

 the opticians' catalogues. For many purposes a magnifier 

 mounted on a plain stand with an extending arm, known 

 as lens-holders or magnifying-stands, will be found very 

 useful. 



It is eminently desirable that the magnifiers employed be 

 of the aplanatized form, consisting usually of three lenses 

 cemented together, similar to Fig. 10, p. 35. Many of 

 them can be had so arranged in the mounting that they 

 can be used either with a dissecting microscope or be carried 

 in a suitable fitting in the pocket. 



Microscopes for Metallurgical Work. — Examination 

 of metal by means of the microscope is a comparatively 

 modern study, but there is probably no iron or steel works 

 of standing that is not equipped with suitable instruments 

 both for observing and photographing. By means of the 

 microscope much information regarding both the chemical 

 constitution and mechanical properties are disclosed, but 

 it is especially valuable for the latter. For instance, the 

 structure of steel varies with the degrees of hardness and 

 the amount of heat to which it has been subjected, and it 

 is possible readily to gain definite information concerning 

 the suitability of the metal for the purposes to which it is 

 to be put by means of the microscope. In the manufacture 

 of guns any defect which may have taken place in the 

 heating or quenching of the steel, which would render the 

 gun unsafe or unsatisfactory, can be discovered before the 

 manufacture is proceeded with. Engineers can detect 

 flaws, blow-holes, defective welds, etc., at an early stage, 

 and avoid the trouble incident to the use of imperfect metal 

 in the finished article. 



Microscopes for this exclusive purpose require no sub- 

 stage or mirror, and although it is customary to employ an 

 ordinary microscope, so that the use may not be restricted 

 to the one especial purpose, two or three well-designed 



