426 MANIPULATION AND PRESERVATION OF THE MICROSCOPE 



their optic axes, (ii.) the parallelism of their planes, (iii.) the setting 

 of their planes at right angles to the optic axis. 



Defining power can only be tested by a critical image. The 

 following is a list of suitable objects of which a critical image is to 

 be obtained, using a solid axial cone of illumination equal to at least 

 three-fourths of the aperture of the objective. 



Very low powers (3-, 2-. and H-inch). Wing of Agrion pul- 

 chelltim $ (dragon-fly). 



Low powers (1 and ). Proboscis of blow-fly. Large diatoms 

 on dark ground. 



Medium powers (^, ^ ^, and low-angled J). Minute hairs on 

 proboscis of blow-fly ; hair of pencil-tail (Polyxenus lagurus) ; 

 diatoms on a dark ground. This last is a most sensitive test ; unless 

 the objective is good there is sure to be false light. 



Medium* potvers (with wide aperture). Pleurosigma formosum ; 

 Navicula lyra in balsam or styrax ; Pleurosigma angulatum dry on 

 cover ; bacteria and micrococci stained. 



High powers (wide aperture and oil-immersion J and -^). The 

 secondary structure of diatoms, especially the fracture through the 

 perforations ; Navicula rhomboides from Cherry field in balsam or 

 styrax ; bacteria and micrococci stained. 



Test with a 10 or 12 eye-piece, and take into account the general 

 whiteness and brilliancy of the picture. 



The podura scale is not mentioned as a test, as it may be very 

 misleading in unskilled hands. One great point in testing objectives 

 is to know your object. Care must be exercised to ascertain by 

 means of vertical illuminator if objects such as diatoms dry on the 

 cover are in optical contact with the cover-glass. Testing objectives 

 is an art which can only be acquired in time and with experience 

 gained by seeing large numbers of objectives. 



In the manipulation of the microscope it is not uncommon to 

 observe the operator rolling the milled head of ihejine adjustment 

 instead of firmly grasping it between the finger and thumb and 

 governing, to the minutest fraction of arc, the amount of alteration 

 he desires. It is undesirable and an entirely inexpert procedure to 

 roll the milled head, and cannot yield the fine results which a delin- 

 eate mastery of this part of the instrument necessitates and implies. 

 To use aright the fine adjustment of a first-class microscope is not 

 the first and easiest thing mastered by the tyro. We have already 

 intimated that the fine adjustment should never be resorted to while 

 the coarse adjustment can be efficiently employed. The focus should 

 always be found, even with the highest powers, by means of the 

 coarse adjustment. It is only a clumsy microscopist who brings his 

 objective by means of the coarse adjustment near the cover-glass and 

 looks at the distance he is off it either by the eye or by the aid of a, 

 hand magnifier, and then completes his work with the fine adjust- 

 ment. In every case the focus ought to be found by the coarse 

 adjustment, and the working distance should be felt by the finger 

 tilting the slide gently against the front of the objective. Also the 

 examination of objects for depth of structure with low and medium 

 powers up to the dry J- or ^-inch objective should be performed by 



