W. HISLOP ON OBLIQUE ILLUMINATION. 67 



converging rays of light at different angles with the surface. 

 Of these rays, those only which are within the angle of total re- 

 flexion for glass, will be transmitted at all ; while those which do 

 pass will be partly polarised, and have the total angle diminished 

 within the transparent medium. (Fig. 4, pi, 4. J Thus, when we in- 

 crease the aperture of our condensers, we often find confusion and 

 fog, not from the quantity of light alone, but because an amount of 

 reflection and dispersion has taken place which we are not able 

 to control. 



There are three remedies for this state of things, by which we 

 may cause rays of light beyond the limit of partial or total reflex- 

 ion to impinge upon our object. In a paper read in 1856, before 

 the Microscopical Society of London, Mr. Wenham describes a 

 method of transmitting a cone of rays at a great angle, by 

 placing beneath the slide a prism or lens, with water, or better, 

 oil of cloves, interposed between the two surfaces. The* object 

 was to get rays through at such an angle, that they would 

 be reflected from the inner surface of the glass cover down upon 

 the object, and so illuminate it as an opaque object ; thus taking 

 advantage of this property of total reflexion for a different purpose. 

 But if we take a similar hemispherical lens, and attach it tempor- 

 arily to the under surface of our slide by means of some medium 

 as nearly as possible of the same density as glass, such as balsam 

 or oil of cloves, we shall be able to transmit a very large angle of 

 light. (Fig. 5, pi. A.) Tliis method will be chiefly available for 

 objects mounted in balsam. 



Another method is to adopt the principle of immersion em- 

 ployed with some of the continental objectives, by applying it to 

 the condenser. A drop of distilled water, or better, of oil of 

 cloves, placed on the upper surface of the condenser, and touching 

 the under surface of the slide, will transmit a pencil of a larger 

 angle than can otherwise be done. 



Another method which I would suggest, is applied to the 

 mounting of the object, and enables any degree of angle of illumina- 

 tion to be used, but has the disadvantage that the objects so 

 mounted are more liable to injury than by the usual method. The 

 structures which we require to examine by oblique light are very 

 minute, such as diatoms. A drop of water containing the organism 

 is spread on the surface of a clean thin glass disc, and dried by 

 gentle heat. The minute particles so dried adhere with consider- 



