MEDIUM-POWER PHOTO-MICROGEAPHY 



81 



special compensating eye-piece, so it is absolutely necessary to use one with this type 

 of objective, and of all the compensating eye-pieces the best results are usually 

 obtained in photography with those specially designed for projection purposes. They 

 are termed " projection eye-pieces." These, with suitable camera length, will nearly 

 always fulfil the requirements of the photo-micrographer, but at times when he wishes 

 over looo diameters to save extreme camera length, it may be necessary to put them 

 aside and use the ordinary compensating eye-piece, as they extend so much higher in 

 power, even up to 27 diameters. This it should be remarked is another superiority 

 of great service with apochromatics ; they bear eye-piecing to almost any amount 

 without producing a " rotten " image. 



Dry and Immersion Objectives. — Whether apochromatic or achromatic, 

 objectives are of two kinds — " dry " and " immersion " — by which is meant that one 

 is used dry as it comes from the maker, and that the other — to insure its best optical 

 performance — needs a drop of cedar-wood oil or some other suitable fluid interposed 

 between the front lens and the cover-glass of the specimen. Both these varieties 

 must be explained, but to do so intelligibly it will be necessary to premise our 

 remarks by considering for a moment, in as few words as possible and in a popular 

 manner without mathematical detail, the subject of refraction of light. 



Light is always supposed to travel in straight lines — "rectilinear propagation" as 

 it is called. Bundles of rays issuing from an illuminant may not be parallel amongst 

 themselves, constituting what is called diverging or converging light ; or they may 

 travel side by side with unerring rectitude — such as we meet with in the light 

 coming from the stars or the sun — when they receive the technical name of " parallel 

 light." The path of any ray in a given medium is always straight until it meets with 

 another medium more or less dense than itself, when, with one exception, it is bent 

 aside, undergoing what is called " refraction." After such bending, however, it will 

 again resume its rectilinear propagation along its new path, until it meets with a 

 fresh medium, when on entrance it will be bent again if the density be different. 

 Change of density then of the medium is the cause of refraction, and this must be 

 held in mind. It will be well now to briefly point out the nature of the alteration of 

 direction brought about by the change of medium, and for purposes of description 

 have resort to Fig. 44. 



Let A B C D represent the outline of a circular vessel, A C being the water line, 

 and B D drawn at right angles to this level passes from B' to D through E — which 

 direction is called the "perpendicular" or "normal," all angles being referred to this 

 line. When the beam is incident along B to E perpendicularly into the new medium, 



