INTERFERENCE LINES. 2o9 



virtual one, is there given a theoretical criterion of discrimination, 

 and in that case a mistake is hardly possible. But in the majority 

 of cases where even the practised microscopist is in doubt this 

 criterion fails us ; each separate case therefore requires a special 

 consideration, and can be explained only by those who have made 

 the observation. 



It is generally advisable to observe optical appearances of this 

 kind with different Microscopes, since the various appearances of 

 the microscopic image, even if they do not always inform us 

 accurately, save us at least from premature conclusions. 



3. INTERFERENCE LINES CAUSED BY THE WITHDRAWAL OF A 

 SOURCE OF LIGHT OF SMALL EXTENT. 



"We have above shown that the approximation of a lumihous 

 point to the focal plane of the Microscope produces a displacement 

 of its objective-image, and in certain circumstances the formation 

 of interference lines. A movement of the source of light in the 

 opposite direction, or, what is equivalent, the elevation of the body- 

 tube, produces, of course, an analogous effect ; the image-point in 

 the microscopic image is likewise displaced, but in the opposite 

 direction. The border of light which the reflecting edge of a 

 mercury-globule produces is moved in this case outwards. It may 

 therefore, according to the peculiarity of the objective, resolve 

 itself into distinct interference rings, or very rapidly become in 

 distinct. Indistinctness will always be met with if the opposite, 

 displacement causes rings, and the formation of rings if it produces 

 indistinctness. 



The example adduced is evidently not the most favourable for 

 the observation of this phenomenon, since the dilating border -of 

 light lies in the illuminated field of view. More suitable objects 

 are formed by fine lines or points in a blackened glass plate (coated 

 with Indian ink or smoked over a candle-flame), or the minute 

 reflected images of a small mercury -globule on a dark ground. The 

 interference lines, if they appear at all, are then extremely sharp, 

 and the peripheral ones exhibit distinct prismatic colours. If we 

 cover up one half of the objective or adjust the mirror laterally, 

 we may easily convince ourselves that the displacement occurs 

 as we have indicated ; by this method also the effect of the eleva- 



