ARRANGEMENT OF APPARATUS, ETC. 115 



ness of cover glass being made according to the 

 method described on page 17, chap, ii.) we bring 

 the object into focus. If the light is too great or 

 deficient, the condenser is slightly moved farther 

 from, or nearer to the object, or the inner tube con- 

 taining the perforated disc may be removed and an- 

 other substituted, and the tube returned to exactly 

 the same position, care being taken to avoid disturb- 

 ing the relative positions of any of the other appar- 

 atus. During its slow removal the eye should be 

 kept on the object, as frequently a slight alteration 

 of the distance of the perforated disc from the glass 

 slip, especially when the light is too great, produces 

 the desired effect. With regard to the proper dis- 

 tance of the condenser from the object; we have 

 found double its focal length, with the bull's eye one 

 inch from the lamp flame, the most favourable posi- 

 tion as a rule. Much, however, depends on the 

 thickness of the glass slip, on which the object is 

 mounted. Some recommend the image of the lamp 

 focussed on the object, this practice is only men- 

 tioned to be condemned. 



Having once found the proper distance for a con- 

 denser with any particular object, the number of 

 divisions marked on the tube of the substage appar- 

 atus, if entered in our note book, enables us to re- 

 peat it ad infinitum. 



The proper quantity, and intensity, of illumination 

 obtained, we proceed to place the camera in position, 

 observing the precautions mentioned when describ- 

 ing a similar process with the one inch. 



i2 



