80 MK. J. SMITH ON THE ORIGIN OF COLOUR 



tion. He says : " I placed, therefore, at that hole a lens 

 or object glass of a telescope which might cast the image 

 of the sun directly on Y without any penumbra at all, and 

 found that the penumbra of the rectilinear sides of the 

 oblong spectrums P T and p t was also thereby taken 

 away, so that these sides appeared as distinctly defined as 

 did the circumference of the first image Y." But what 

 does this prove ? It only proves that there was not so 

 much penumbra on the sides of the spectrum when the 

 lens was used, but it does not prove the same as to the 

 ends, the direction in which the light is refracted by the 

 prism. There is little dispersion or refraction on the sides 

 even at first ; the dispersion is transverse, or towards the 

 thick part of the prism, not longitudinal, or towards the 

 ends of the prism. But how could Newton himself be 

 satisfied with this experiment when a little farther on in 

 his work he elaborately attempts to prove the impossibility 

 of forming an achromatic glass ? He says ; " Seeing, 

 therefore, the improvement of telescopes of given lengths 

 by refractions is desperate, I contrived heretofore a per- 

 spective by reflection, using instead of an object glass a 

 concave metal. ^^ This admission of itself proves that 

 Newton was not experimentally in a condition to say that 

 the object glass removed all penumbra. It is known that 

 light, in passing the edge of a solid, always causes a 

 penumbra greater or less according to the distance of the 

 luminous body from the object, or of the object from the 

 place on which the image is received. This penumbra 

 may be either a graduated grey, or coloured according to 

 circumstances. It is these circumstances which we are 

 now investigating. 



123. I shall quote another experiment of Newton^s in 

 reference to this point [Optics, p. 102) : 



"Experiment 3. Such another experiment may be easily 

 tried as follows : Let a broad beam of the sun's liaht 



