"270 TECHNICAL MICROSCOPY. 



mination is found. As a general rule, striations, systems of lines, 

 <fec., are seen most distinctly when the light is incident as obliquely 

 as possible ; the relative position of mirror and object must always 

 be arranged so that the direction of the incident rays is at right 

 angles to the strife. 



As in most cases the source of light is limited, or unequally 

 luminous, the concave mirror gives (for reasons previously stated) a 

 more intense light than the plane mirror i.e., it involves, with 

 equal aperture of the incident cone of light, a greater brightness 

 of the field of view. For high amplifications the concave mirror 

 affords, therefore, decided advantages ; for the lower amplifications 

 its superiority might easily be exaggerated, because a field of view 

 too glaringly bright soon fatigues the eyes. If the mirror is suffi- 

 ciently large and can be adjusted near the object, the application 

 of illuminating lenses or condensers is in most cases superfluous. 

 Condensers are useful when the aperture of the incident cone of 

 light is required to be enlarged. 



If we use reflected light for the illumination, the Microscope 

 should be placed as near as possible to the window, to obtain a 

 moderately extensive source of light. With amplifications exceed- 

 ing 100 to 120 linear, it is advisable to employ an illuminating 

 lens, or the Licberkukn mirror. In certain cases, also, sun- 

 light may be utilized by inclining the Microscope so that the rays 

 may strike the object directly. We doubt the accuracy of the 

 statement of Schacht, that the three series of lines of Plcurosiyma 

 angulatum may with equal amplification be more distinctly seen in 

 this manner than with ordinary transmitted light. We do not 

 recommend this method of illumination for transparent objects. 



Illumination by lamp-light, when subdued by ground-glass or 

 cobalt-blue glass, is useful for many experiments, though it 

 always strains the eyes more than day-light the latter is also far 

 more agreeable. Hence, for the preservation of the sight, prolonged 

 microscopical observations should be limited to the hours of day- 

 light. But in England and generally in Northern Europe, where 

 the cloudy day -light is only of short duration, artificial illumina- 

 tion must be utilized. Under these circumstances the focal length 

 of the concave mirror or of the illuminating lens must be duly 

 considered, so that the convergence of the incident rays may 

 reach its maximum ; this matter hardly needs further explanation 

 after what has been stated. In the various opticians' catalogues, 



