THE OPTICAL EQUIPMENT 41 



compensating oculars, except that they are of low power, only 

 amplifying the image from two to six times, and are constructed 

 to project an image to a considerable distance. No ordinary 

 ocular is designed to do this, although in fact it often happens 

 that it will do so without any serious loss of definition. 

 Projection-oculars do not differ from the ordinary eye-piece in 

 the way in which they fit into the tube of the microscope, and 

 as already stated in reference to compensating oculars they 

 are designated by a number which indicates their magnifying 

 power. A projection-ocular, No. 2 which to obtain this ampli- 

 fication should be used on a short tube will become approxi- 

 mately a No. 3 when used on the long tube, and no difference 

 in the quality of the image is appreciable when used under either 

 condition. In each ocular is a diaphragm which is adjustable in 

 position within the ocular itself by a spiral movement (Fig. 12). 

 By turning a milled head at the top of the ocular the image 

 of this diaphragm can be exactly focussed on the ground- 

 glass focussing-screen of the camera, and this operation must 

 be carefully performed with each alteration of camera -length. 



The writer is of the opinion that where ordinary achromatic 

 objectives are used at least those of the older types better 

 results are often obtained by not using an ocular at all for 

 photographic work. The great disadvantage of this method 

 is that a much longer camera may be required to obtain a 

 given magnification ; since it is evident that no increase in 

 the magnification occurs, such as would of course result from 

 using an ocular. It becomes necessary, therefore, to have 

 a camera which is capable of extension to as much as four or 

 five feet in length ; but where this is not an insuperable 

 objection, and in cases where apochromatic objectives are not 

 available, it is often a satisfactory method. 



It must not be overlooked that with any ordinary objective, 

 which has an appreciable outstanding aberration of any de- 

 scription, an ocular only tends to increase the imperfection of 

 the resulting image. It is evident that an ocular magnifying, 

 say, ten times, would render much more apparent any errors 

 in the objective than would a lower power one, and in any case 

 it is therefore advisable in photographing to use oculars of low 

 power, the necessary further amplification of the image being 

 obtained by camera extension. 



