42 PBACTICAL PHOTO-MICBOGKAPHY 



With achromatic objectives of recent construction e.g. with 

 those which are sometimes designated as semi-apochromatic 

 compensating oculars work extremely well. In many of these 

 lenses an approximation to the correction of the apochromatic 

 lenses is obtained, and they are to some extent dependent on 

 the oculars for obtaining the best results. Two firms have 

 introduced a type of ocular which may be used with either 

 achromatic or apochromatic objectives. These are called by 

 Messrs. J. Swift & Son the ' Universal,' and by Messrs. W. 

 Watson & Sons the * Holoscopic,' eye-pieces. While they are 

 of considerable use for visual work where a mixed series of 

 objectives is in use, they cannot be recommended for photo- 

 graphic work of a critical nature. The alteration of correc- 

 tion is effected by altering the distance between the eye-lens 

 and field -lens of the ocular ; but at best they can only claim 

 to be a compromise between the two recognised forms of 

 ocular already described. A rough method of determining at 

 sight whether an ocular is a compensating or an ordinary 

 one, is to hold it up to the light preferably to the sky : the 

 image of the diaphragm then seen by direct observation will 

 be fringed with a double band of red and orange ; or, with 

 the very low-power oculars, a yellowish edge only may be 

 apparent. With ordinary oculars the same observation will 

 usually disclose a well-defined blue fringe. Sometimes, to 

 make this colour -fringe visible, the angle of incidence of 

 the light in relation to the ocular may have to be varied, 

 but usually it is at once apparent. 



A point of considerable importance in visual work is the 

 most suitable combination of objective and ocular. In photo- 

 graphic work this question does not arise, perhaps, to such an 

 extent, as the oculars used are usually of lower power ; but even 

 then, under certain conditions, it is of importance. It is an 

 obviously simple method for obtaining increased amplification 

 of the image to use a higher power ocular. Such a proceeding, 

 however, if carried to excess, introduces many difficulties. In 

 visual work the beam entering the eye with a high-power ocular 

 is very much narrower than is the case with a low one, and 

 consequently any small particles of dust lying within the 

 ocular itself or the moving specks in the fluid of the eye become 

 painfully visible. In photographing, these specks are most 



