38 PEACTICAL PHOTO-MICEOGEAPHY 



that made by Messrs. W. Watson & Sons. The advantages put 

 forward by the makers are that it is very thin and only increases 

 the tube-length by 12-5 mm. ; that it is simple and reliable 

 in use, and as it is made of magnalium it is very light, and so 

 adds but little to the weight to be carried by the microscope 

 tube. The turning of the milled head results in the inversion 

 downwards of a pair of jaws having a screw-thread cut upon 

 them. The objective is placed in the aperture, and directly the 

 milled head is released, the jaws are carried back and engage 

 the threads of the objective. It therefore acts as a permanent 

 attachment to the body-tube of the microscope, and each 

 objective may be inserted without the necessity of screwing it 

 in, as is the case when some such arrangement is not in use. 

 Of course it does not admit of exact centra tion of each objective 

 as does the Zeiss changer, but in use it would certainly be 

 found to be more exact than a multiple nose-piece. With 

 moderate- or low-power objectives this would not be such a 

 great objection, as small errors of centration are not so 

 apparent. 



The Ocular or Eye-piece. An important optical part, and 

 one which in the case of the apochromatic objectives is an 

 essential portion of the magnifying system, is the ocular. Its 

 function is to pick up and transmit to the eye or, in the 

 case of projection-oculars, to the photographic plate the 

 image already formed by the objective, always amplifying it 

 to a greater or less extent. 



There are many types of oculars, but for microscopic work 

 the two almost universally used are those known as the 

 Huyghenian and the Bamsden. These may again be divided 

 into ordinary and compensating : the latter distinction depending 

 on whether they are for use with achromatic or apochromatic 

 objectives. Both of the first-mentioned types occur in the Zeiss 

 series of compensating oculars the lower power oculars being 

 of the Huyghenian and the higher ones of the Bamsden type. 



In referring to the ordinary oculars, which are suitable for 

 use with achromatic objectives, some makers have adopted an 

 arbitrary method of description simply designating them by a 

 letter, which gives no indication of their amplifying power, 

 although custom has decided approximately what each letter 

 may mean. They are made in a series of regularly increasing 



