150 PKACTICAL PHOTO-MICKOGKAPHY 



useful in enabling one not conversant 'with the appearances 

 obtainable to appreciate what may be done by alteration of 

 position of the various parts in use. 



The Use of Objectives and Oculars. In ordinary observa- 

 tional work the use of the low-power objective presents no 

 particular difficulty ; it is only necessary to screw the objective 

 into its proper position on the microscope body-tube, use it in 

 conjunction with a suitable eye-piece, and focus the objective 

 oil the object, when an image will appear. As a practical 

 point, it is always advisable with low or medium powers to rack 

 down the objective with the coarse-adjustment of the microscope, 

 so that the front of the objective is in close proximity to the 

 object. To bring the object into focus the motion of the body- 

 tube is therefore away from the object. This is only to avoid 

 the possibility of any injury to the specimen or the objective 

 by racking down too far by means of the coarse-adjustment. 

 With high-power objectives, and particularly oil-immersions, this 

 is hardly practicable, as the working distance is already so small 

 that to bring the objective still lower down is a matter of 

 considerable difficulty. The safest way therefore is, having 

 immersed the objective in oil, to carefully focus down until 

 some particle of dust on the top of the cover-glass is in the field 

 of view, and then by means of the fine- adjustment to work down 

 until the object is seen. Incidentally it may be remarked that 

 it is more satisfactory to place the cedar-wood oil for immersion 

 purposes on the front of the objective itself, and then to lower 

 this down on to the object, than to place the oil on the object- 

 slide, as is so generally done. This is perhaps a small point, but 

 it has a definite bearing on the work in hand, as it ensures the oil 

 being in the proper position on the object, assuming that some 

 preliminary centration has been effected with a lower power 

 objective; and, again, it prevents the possibility of too large a 

 quantity of oil being used. When the microscope is thrown into 

 the horizontal position if there is any considerable excess of oil 

 it tends to flow away, with the result that if it is very fluid the 

 oil may leave the objective and specimen entirely ; and, as this 

 may occur during an exposure, it would mean that the photo- 

 graphic result would be entirely spoilt. 



Before inserting an ocular to use in conjunction with the 

 objective it is important to see that the tube-length is that for 



