300 THE NORTHERN MICROSCOPIST, 


consequence of the power of accommodation or self-correction 
having been lost with age. 
In microscopic drawing, as with the camera lucida, the perspec- 
tive may be misrepresented, in consequence of astigmatism, and thus 
endless disputes may arise even amongst the most careful observers. 
We have now to deal with errors of refrangibility, and it will 
probably have been assumed that the eye apparatus is entirely 
corrected for colour. ‘This is not the case, however, except when 
an object is in exact focus, and the reason that the error due to 
refrangibility remains practically unnoticed is that the distance be- 
tween the focal point of the red and violet rays is extremely small. 
The error due to refrangibility may be noticed by means of the 
concentric circles already shown you; by bright daylight adjust 
the eyes to some object twelve inches away, and without moving the 
eye insert at a distance of four inches the card inscribed with black 
circles, when a yellow and blue colouring will be plainly discerned. 
In order that 
you may tho- 
roughly under- 
stand the error of 
refrangibility, the 
picture afforded 
by the passage 
of a solar ray 
through a prism 
of glass is thrown 
upon the screen, 
the rays are de- 
Noe NE flected unequally, 
the red least and 
Fig. 30. the violet most. 
(Fig. 30.) 
It may be advisable here to state that the degree of dispersion 
of the rays of white light depends upon the medium through which 
the ray passes, and this amount of dispersion is measured by the 
distance the most prominent dark lines in the spectrum are from 
each other. ‘The diamond disperses much less than crown glass, 
while the deflection of the ray is greater; but this is a subject it is 
not possible to enter upon this evening. 
I now show you a diagram illustrating the effect produced upon 
a ray of white light by a double convex lens, but. you must not 
consider the subject can be practically studied as easily as the dia- 
gram would indicate. The chromatic and spherical aberrations 
are very complicated phenomena, and although they may be dia- 
grammatically represented upon a screen, still the picture seen by 
a double convex non-achromatic lens of glass is not foreshadowed in 
any way in the diagram. Zits was shown by a coloured transparency. 
SS 
bs . 
a 

