Observations on Monochromatic Light. 147 



solation that we still surpassed them in microscopical improve- 

 ments. 



" On the subject of telescopes," says Mr Coddington, u I 

 have done little more than detail and explain to the student 

 the improvements which the talents and industry of Huygens, 

 Dollond, and Ramsden, had long since made available in prac- 

 tice. With regard to the microscope, I may perhaps lay claim 

 to a little more. That instrument had, till very lately, been 

 almost entirely neglected since the days of Campani, and I am 

 inclined to doubt whether the intellectual and manual labour 

 which has within a few years been bestowed upon it have 

 been on the whole profitably directed, since they have left 

 wholly untouched a source of error perhaps the most import- 

 ant of all, namely, the obliquity of the refraction at the ob- 

 ject-glass." 



We entirely agree with Mr Coddington in the opinion, that 

 the microscope has been almost entirely neglected since the days 

 of Campani ; but we differ widely from him in his estimate of 

 the value of the intellectual and manual labour which has been 

 lately bestowed upon it. — We regard the last twenty years as 

 the golden age of microscopical improvement ; and in proof 

 of this position, we shall enumerate some of the intellectual and 

 manual labours by which this period has been so remarkably 

 distinguished. 



1. Dr Wollaston's introduction of centrical pencils by 

 means of two hemispheres separated by a diaphragm. 



2. Dr Brewster's grooved or excavated sphere, very inap- 

 propriately called the bird's-eye lens by Mr Coddington. 



3. The catoptric hemisphere, by which the magnifying 

 power of a hemisphere is doubled. 



4. Mr Herschers double lenses free from spherical aberra- 

 tion. 



5. Dr Wollaston's microscopic doublet. 



(5. Dr Wollaston's improved method of illumination. 



7. Introduction of monochromatic light. 



8. Introduction of microscopes made of garnet, sapphire, 

 and diamond. 



9. The improved Amician reflecting microscope. 



10. Selligue's idea of combining achromatic object-glasses. 



