MANAGEMENT OF THE MICROSCOPE. 151 



fation were fully made known, it would generally appear that the sta- 

 ility and completeness of the conclusions finally arrived-at had only been 

 attained after many modifications, or even entire alterations, of doctrine. 

 And it is, therefore, of such great importance as to be almost essential to 

 the correctness of our conclusions, that they should not be finally formed 

 .and announced until they have been tested in every conceivable mode. 

 It is due to Science that it should be burdened with as few false facts and 

 false doctrines as possible. It is due to other truth-seekers that they 

 .should not be misled, to the great waste of their time and pains, by our 

 errors. And it is due to ourselves that we should not commit our repu- 

 tation to the chance of impairment, by the premature formation and 

 publication of conclusions, which may be at once reversed by other ob- 

 servers better informed than ourselves, or may be proved to be fallacious 

 at some future time, perhaps even by our own more extended and care- 

 ful researches. The suspension of the judgment, whenever there seems 

 room for doubt, is a lesson inculcated by all those Philosophers who have 

 gained the highest repute for practical wisdom; and it is one which the 

 Microscopist cannot too soon learn, or too constantly practise. Besides 

 these general warnings, however, certain special cautions should be given 

 to the young Microscopist, with regard to errors into which he is liable 

 to be led, even when the very best instruments are employed. 



152. Errors of interpretation arising from the imperfection of the 

 focal adjustment are not at all uncommon amongst young Microscopists. 

 "With lenses of high power, and especially with those of large angular 

 aperture, it very seldom happens that all the parts of an object, however 

 minute and flat it may be, can be in focus together; and hence, when the 

 focal adjustment is exactly made for one part, everything that is not in 

 exact focus is not only more or less indistinct, but is often wrongly repre- 

 sented. The indistinctness of outline will sometimes present the appear- 

 ance of a pellucid border, which, like the diffraction-band, may be mis- 

 taken for actual substance. But the most common error is that which 

 is produced by the reversal of the lights and shadows resulting from the 

 refractive powers of the object itself; thus, the bi-concavity of the blood- 

 disks of Human (and other Mammalian) Blood occasions their centres to 

 appear dark when in the focus of the Microscope, through the divergence 

 of the rays which it occasions; but when they are brought a little within 

 the focus by a slight approximation of the object-glass, the centres appear 

 brighter than the peripheral parts of the disks. An opposite reversal 

 presents itself in the markings of certain Diatoms, which, like Pleuro- 

 sigma angulatum, present, when ex- 

 actly focussed, the aspect of rows of 

 hemispherical beads (Fig. 166, A). 

 When the surface is viewed a little 

 inside the focus, its aspect is that 

 shown at A, Fig. 115; whilst, when 

 the surface is slightly beyond the 

 focus (B), the hexagonal areolae are 

 dark, and the intervening partitions 



light. The experienced MicrOSCOpist False hex agonal areolation of Pleuro- 



will find in the Optical effects produced Sigma angulatum, as seen in a Photo- 

 , ... **m i -i graph magnified to 15,000 (fcameters. 



by variations of focal adjustment the 



most certain indications in regard to the nature of such inequalities of 

 surface as are too minute to be made apparent by the use of the Stereo- 

 scopic Binocular. For superficial elevations must necessarily appear 



