20Q E. M. NELSON ON DARK-GROUND ILLUMINATION. 



make a doublet with a sufficiently small denominator. The only 

 form it could have would be that of a hyper-rapid portrait lens. 

 If these are tried one against the other, the difference, if any, 

 would be in favour of the non-achromatic doublet, which would 

 pass more light than the photographic combination, because it 

 works at f/1'2, and is therefore more rapid than any portrait 

 lens made. 



Place the lamp on the left-hand side of the microscope and at 

 10 or 12 inches' distance from the mirror; remove substage 

 condenser and objective from the nose-piece ; place a low-power 

 eye-piece in the top of the tube. Incline the plane mirror and 

 adjust the height of the lamp so that the bright part of the 

 flame falls centrally upon it. Now incline the mirror so that 

 the full beam is reflected up the microscope tube. From a 

 distance of 6 or 8 inches look at the bright spot of light at 

 the eye-lens. This should be an evenly illuminated bright disc ; 

 if it is not, a slight movement of the mirror or lamp will make it 

 so. When this adjustment is completed neither the lamp nor 

 mirror must be disturbed. Keplace condenser and object-glass and 

 proceed to the work in hand, centring the substage condenser and 

 focusing it in the usual manner. If an unbroken disc of light 

 has been obtained at the eye-lens, an unbroken disc of light will 

 be focused on the object on the stage by the substage condenser ; 

 therefore, when a suitable stop, one not too large, has been 

 placed below the substage condenser, a dark-field of maximum 

 brightness, for that set of apparatus, will be secured. 



This method of arranging the lamp, bull's eye and mirror is 

 important, because when any of these new dark-ground illumi- 

 nators with fixed stops are employed an image of the light cannot 

 be seen, and therefore the microscopist can know nothing about 

 the condition of his illuminating beam ; whether, for instance, it 

 is central, or whether the disc of light is not broken by dark 

 iireas. 



A dark-ground illuminator with a fixed stop should always 



