GLASGOW SOCIETY OF FIELD NATURALISTS. 123 



colour and composition of the light. The undulations of the rays 

 of light exciting vision vary between l-481st and l-764th of a 

 billionth of a second of time, and they take place in two planes 

 perpendicular to the direction of the ray. The light, or the undu- 

 lations of the luminiferous ether, emitted from a luminous body, 

 travel in straight lines, but when they pass through a medium 

 falling upon its surface not perpendicularly they are deviated from 

 their course or refracted. They are always refracted when passing, 

 at an angle of less than 90 deg., from one medium into another 

 of more or less density. The amount of refraction is dependent 

 upon the relative densities of the media, and is further conditioned 

 by their form, in a microscope by the shape and the material of 

 the various lenses. In optical instruments the curvature of the 

 lenses is spherical, being the only form that can be given by grind- 

 ing with the requisite amount of truth. Unfortunately, spherical 

 convergent lenses do not bring all the rays of light which pass 

 through them to one and the same focus. The peripheral rays 

 have a shorter focus than the central rays, and the extreme 

 distance between the various foci of a lens is called spherical 

 aberration. The consequence is, that an object-glass whose 

 spherical aberration is not corrected will not show the various 

 portions of the surface of a plane object to be in focus at the same 

 time ; it will either only show the central portion defined or the 

 peripheral poition alone, and this fault in an object-glass is called 

 want of flatness of field. 



In the best objectives of -l-inch focus the spherical aberration is 

 computed to be still the l-50,000th of an inch, a small fault, it 

 is true, but sufficient to prevent the resolution of markings several 

 times closer than l-50,000th inch. 



The second difficulty is the chromatic aberration, which results 

 from the unequal refrangibility of the different colours of which 

 the white light is composed, so that they are not all brought to 

 the same focus, even by a lens free from spherical aberration. If 

 you look through a microscope in which the chromatic aberration 

 of the lenses is not corrected, you will find that the image has an 

 iridescent border of red, yellow, and blue. In fact, you see one 

 image for each of the colours of the spectrum, but as all these 

 images overlap in the centre, they produce white light. Not so at 

 the outlines. The correction of this chromatic aberration is 

 effected by combining in the construction of lenses two media of 



