70 PRACTICAL PHOTO-MICKOGKAPHY 



lens of short focus which is placed so that it projects a practi- 

 cally parallel beam of light into the posterior combination of 

 the condenser. The plane side of the lens should be turned 

 towards the light. It is usually supplied with vertical and 

 horizontal adjustment so that it can be placed in correct 

 alignment with the other portions of the optical system. It 

 is very useful for low powers, and for work not exceeding 

 one hundred diameters it will be found of service, but, owing 

 to the fact that it is uncorrected, it is not suited for use with 

 high powers or for critical work. 



An improved type of this appliance designed by Mr. E. M. 

 Nelson may with advantage be substituted for the ordinary 

 form. In this the condensing system is composed of two lenses 

 so that spherical aberration, inherent in the single bull's-eye, 

 is considerably reduced, and the illumination obtained by it 

 is of increased brilliancy. It is usually supplied with centreing- 

 screws similar to those used for sub-stage condensers, as it is 

 much more sensitive to errors of centration than the ordinary 

 type. It is important that the plane or concave side should 

 be towards the source of light, as already indicated for the 

 ordinary bull's-eye. It has proved to be a useful accessory, 

 and will fulfil all the ordinary requirements of a photo- 

 micrographer where the more elaborate condensing systems 

 to be described are not available. 



The Watson-Conrady collecting-lens system is one that has 

 some interesting characteristics. In this the lenses are purposely 

 kept small, on the ground that with large lenses, especially those 

 uncorrected for spherical and chromatic aberration, a great 

 amount of the light passed cannot possibly be utilised in the micro- 

 scope. The system consists of a carefully corrected achromatic 

 lens of 2J inches diameter and 3J-inch focus, by which a sharp 

 and enlarged image of the source of light can be produced and 

 projected in the position of the iris-diaphragm, producing the 

 best effect in the sub-stage condenser. As this lens comes so 

 close to the source of light and accordingly is apt to get very 

 hot, it cannot be cemented with Canada balsam in the usual 

 way, but has to be put together with glycerine. The glycerine 

 will resist the heat for a considerable time, and in the event 

 of its evaporating off can be replaced by the user. 



If reference is made to the figure illustrating the parts of 



