22 THE GENERAL PRINCIPLES OF THE METHOD 



a specimen without disturbing either the microscope 

 or the specimen. Having obtained the negative, the 

 camera is removed in a moment and the examination 

 of the particular cell or specimen under observation 

 can be immediately proceeded with in the usual way. 

 The old forms of cameras which necessitated the moving 

 of the microscope or the specimen are not useful for 

 recording specimens of living cells. An instrument 

 is required capable of being immediately connected 

 with the microscope as it stands, so that two or three 

 records of the same cell may be taken before it dies or 

 becomes achromatic and vanishes. It is necessary to 

 use a powerful light, and the light itself will kill the 

 cells if they are exposed to it for very long. For this 

 reason we employ a powerful light for the photography 

 and another for the eye work, but each of them fixed 

 and capable of being used independently of one another. 

 The inverted gas-burner above referred to, being placed 

 at a distance of two feet above the mirror, gives a soft, 

 indirect illuminant for ordinary work, the other being 

 a powerful electric Nernst burner, which is placed 

 behind (that is, underneath) the mirror. When a 

 photograph is to be taken the mirror is swung aside, 

 and the light from the Nernst lamp replaces that from 

 the gas one. 



The microscope is fixed on the bench and tilted 

 at an angle of about 45 from the vertical. All 

 the microscopes which we use are bolted perma- 

 nently on to the bench, and they can only be 

 moved with the aid of a screwdriver. The instru- 

 ments are not placed vertically, but are tilted at 



