ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



497 



On this point we will observe that the student will find his range 

 of experience much increased by varying tbe position of the mirror so 

 as to make the illumination more or less oblique. The differences 

 between the positions of the draw-tube required to obtain the more 

 perfect definition will thus be much more plainly ajjpreciable by the 

 untrained eye, and he will thus learn to discriminate at a glance when 

 he is obtaining the best images his objectives will produce. 



Further, this method of practice should also be adopted in con- 

 junction with the correction-collar of the objective, which should be 

 turned slowly from end to end of its range in one direction, and then in 

 the other whilst following the varying focus by the other hand on the 

 fine-adjustment. The eye and the hand will thus be trained to the 

 skilful employment of the Microscope, a matter which has been far too 

 much neglected hitherto. 



It is a subject of common observation by opticians that the great 

 majority of Microscopists have no practical training in the use of a 

 correction-adjustment in improving the quality of the image under 

 varying conditions of the illumination and with different thicknesses of 

 cover-glass. Through neglect of such points the student drifts into 

 regarding the correction-adjustment as useless ; hence, he too frequently 

 contents himself with mediocre definition, when his Microscope is 

 capable of superior work if only properly handled. 



Adjusting an Objective for the Thickness of the Cover-glass. — 

 In a description of their " National " Microscope, Messrs. E. and J. Beck 

 give directions for adjusting an objective, which are conveniently arranged 



Fig. 87. 



Fig. 88. 



Fig. 89. 



Fig. 90. 



Fig. 91. 



Fig. 92. 



for the use of the student microscopist, and with these they give the 

 following figures showing the appearance of a Podura scale when (fig. 87) 

 1888. 2 M 



