MICROSCOPES. 329 



made somewhat less intolerably bad by the use of more lenses 

 than two. The whole instrument was all but unfit for scientific 

 research, and would now be looked upon as very little better than 

 a toy to please those who are amused at seeing a small object 

 look very large, regardless of the accurate definition of minute 

 detail. The theoretical and practical difficulties that had to be 

 overcome in developing the best modern compound microscopes 

 from this embryonic condition were so great that, until within 

 the last fifty or sixty years, the very possibility of success was 

 doubted by the highest authorities in optical science. 



OBJECT-GLASSES. 



Object-glasses should be so constructed that after having 

 passed through the lenses of the eye-pieces used to view the 

 image, all the different rays of light proceeding from a luminous 

 point may enter the pupil of the eye so nearly parallel as to form 

 on the retina an image of the point free from surrounding haze or 

 colour. The possibility of doing this depends on the fact that 

 the extent to which the red and blue rays are separated by passing 

 through different kinds of glass does not vary directly as the 

 extent to which they are bent, or, to use the technical expression, 

 their dispersive power does not vary directly as their refractive 

 power. The result of this is, that we may construct at pleasure 

 direct vision prisms that do not bend the light, but separate the 

 constituent rays of the spectrum, or prisms that bend all the 

 rays at nearly the same very considerable angle, so that though 

 the vision is indirect, the object is free from false colour. This 

 of course is what is necessary in the case of lenses. We require 

 to have the light bent very considerably, and yet not to have the 

 red and blue rays separated by being unequally bent. This is, 

 however, not the only difficulty. The perfect performance of the 

 instrument also depends on the different rays of light being 

 brought to the same focus by both the centre and the circum- 



