FOCAL ADJUSTMENT OF MICROSCOPE. 779 



are so numerous as to form an extensive shower or cloud. The pearly 

 threads are of different lengths ; some of them very short, others 

 stretching across the whole field. Not unfrequently some of them end 

 abruptly in a sort of bulb. The globules or pearls forming the threads 

 or rosaries Jseemed joined together merely by apposition, without being 

 contained in any tube. Sometimes, however, the globules are rather 

 indistinct, and then the threads approach to a tubular appearance. 

 The globules are always iu single rows. They appear destitute of any 

 nucleus. They are not all of one diameter, but are all smaller than 

 the globules of the iusulo-globular spectra. I have not satisfied myself 

 that aU the pearly threads occupy the same plane, although it is very 

 evident that they are behind the insulo-globular spectra. 



" That portion of the pearly spectrum which appears in the centre 

 of the field of view has but little real motion, less perhaps than the 

 watery spectrum which is seen beyond it. Both partake, of course, 

 in the motion of the eyeball ; and this gives to both a' wide apparent 

 motion. But if the field be examined towards its circumference, or if 

 the eye be suddenly rotated upwards, other pearly spectra appear, 

 which it is diflBcult or impossible for the observer to bring directly 

 before him; and which, when he succeeds in some measure in doing 

 so, quickly subside again out of view, partly by a real motion of their 

 own, partly by a wide apparent motion, owing to their obliquity in 

 respect to the axis of vision. It is these last spectra, chiefly, which 

 produce the pearly muscse volitantes." 



There are also various optical phenomena caused by refraction, and 

 which are necessary to be attended to. They depend, for the most 

 part, upon a bad adjustment of the focus, or illumination of the object. 

 The appearances are also most frequently associated with an increase 

 of the magnifying power, and especially with the use of powerful eye- 

 glasses. Large grains of potato-starch, pollen-grains, the thickened 

 substance of woody tubes, and the cells of cartilage, are among the 

 most common objects which exhibit such optical phenomena, which 

 consist in a feeble and generally yellowish colouring of the edges of 

 the objects when seen vnith particular foci. 



Focal Adjustment or the Miceoscopb. — The regulation of 

 this adjustment is based on the fact that the microscope can only 

 afibrd a view of one surface of an object at any given time, so that 

 nothing is distinctly seen which lies above or below such a focal plane 

 at that time ; and the more flat the field of vision, the clearer and 

 better will be the view of objects in that plane if the adjustment is 

 correct. The more perfect the object-glass, and the greater the angle 

 of aperture,* the more exact is this focal plane, and the more sensitive 



* The angle of aperture is that made by two lines from opposite ends of the aperture of 

 the object-glass with the point of focus of the lens. A glass with a large angle of aperture 

 shows objects clearly. The angle varies usually from 50* to 100". Many glasses, however, 

 are made with a much higher angle. Eoss makes glasses of 170* of angular aperture. 

 These are useful for observing minute organisms, such as Diatoms. 



