72 PHOTOMICROGRAPHY 



other apparatus used between the tube and objective 

 must be taken into account with the tube length. 



Oculars or Eyepieces. There are three kinds of eye- 

 piece in general use for photomicrography. Ordinar}^ 

 eyepieces of the Huyghenian type, Compensating eye- 

 pieces, which are made of both the Huyghenian and 

 Ramsden types, and Projection eyepieces. 



Huyghenian Eyepieces. The Huyghenian is com- 

 posed of two plano-convex lenses with their convex 

 surfaces downwards (as shown in Fig. 22), and it is 

 interesting to note that chromatic correction has been 

 achieved, to a remarkable extent, by the use of two un- 

 corrected lenses. This is rendered possible by so con- 

 structing them that the errors caused by the field-lens 

 are in part compensated by those of the eye-lens, com- 

 bined with the fact that although the individual blue 

 and red images are of different sizes, they fall at such 

 distances from the eye that the smaller one exactly 

 covers the larger. This arrangement demands that the 

 ocular should be used in the way for which it was designed 

 to get accurate corrections, and not for projection, but 

 in practice it is found that well-made eyepieces of this 

 type can be used with great satisfaction for photomicro- 

 graphy. 



Compensating Eyepieces. Corrected lenses are used 

 in the Compensating eyepieces, which are so made 

 as to exactly neutralise or compensate the errors of 

 under-correction which have been left in the apochro- 

 matic objectives. They also are primarily intended 

 for visual work, but the photographs they give in 

 conjunction with apochromatic or achromatic objec- 

 tives of high power leave practically nothing to be 

 desired. 



Projection Eyepieces. Projection eyepieces are specially 

 made for photomicrography to give an image on a 

 screen situated beyond them. They are intended for 



