353 



PHYSIOLOGICAL PHYSICS. [Chap. xxvm. 



magnifying lenses were very great, difficulties arising 

 from the aberrations of sphericity, which rendered the 

 object difficult to focus with good definition, and from 

 the error of chromatism due to the dispersive power 

 of the lens. These prevented much progress being 

 made in the improvement of the instrument. One 

 improvement consisted in the employment of two 

 plano-convex lenses instead of one, the convex sides 

 being directed towards the eye, the focal length of the 

 one next to the eye being three times that of the 

 lens next the object. This was called Wollaston's 

 doublet. It diminished the amount of the aberra- 

 tions, and specially so when, later, a diaphragm was 

 interposed between the two lenses. 



The compound microscope consists, in its 

 simplest form, of two lenses, one next the object, 



called the object-glass^ and one 

 next the eye called the eye-glass. 

 The action of the two is shown in 

 Fig. 161, where LL' is the object- 

 glass, and MM' the eye-glass. AB is 

 a small object placed beyond/, the 

 focal distance of LL'. By the action 

 of LL' a real inverted image of 

 AB is formed on the other side of 

 LL', viz. ab. Rays from ab diverge 

 towards the eye-glass MM', which 

 is so placed that its focal distance 

 is t /i Thus, ab is within the prin- 

 cipal focus of MM'. Rays from ab 

 will, therefore, still diverge after 

 P assin g through the lens MM', but 

 will, by the action of the media of 

 the eye of the observer placed beyond MM', be brought 

 to a focus on the retina. The image on the retina will 

 be referred in the direction of the divergent rays enter- 

 ing the eye, in the direction, that is, of the dotted lines, 



