60 Improvements in the Microscope. 



It is not known that purple light has ever been used before for the 

 purpose of increasing the real power of a microscope : indeed it ap- 

 pears to be of no sensible advantage, unless the compound lens will 

 show a more minute object than can be seen with any single lens. 

 One of my sextuple object-glasses has a focal distance of 0.07 of an 

 inch ; when this is used there is an advantage in the application of 

 purple light, and it is believed that this combination will exhibit a 

 smaller object than any other optical instrument hitherto made. 



The superiority of this microscope over those constructed in Eu- 

 rope (as before mentioned,) consists, first, in its having less seconda- 

 ry aberration of figure; second, in having fewer reflecting surfaces; 

 third, in having a shorter focal distance ; and fourth, in the applica- 

 tion of purple light. 



A much simpler combination than the above was suggested, for de- 

 stroying all the spherical aberration, by J. F. Herschel, Esq. consist- 

 ing of a double-convex lens with radii as one to six, placed in contact 

 with a meniscus. It v/as believed that this combination would be 

 nearly free from the errors of color, when purple light was used, and 

 quite free from spherical aberration. But after Investigating it both 

 practically and theoretically, I found with regret, that when the thick- 

 ness of the lenses vs^as such as is necessary to have in an object-glass 

 of high power, that the marginal rays, (although the aberration of fig- 

 ure was much reduced,) had a shorter focus than the central rays. 

 This combination might probably be improved, when its aperture is 

 large, by adding a plano-concave lens of the same kind of which the 

 other two lenses are made, having its concave side towards the par- 

 allel rays, and by changing the curves of the meniscus so as to de- 

 stroy the aberration of figure. 



A very simple microscope, mentioned in the American Journal of 

 Science and Arts, Vol. 17, p. 362, was proposed by Dr. Wollaston 

 consisting of two plano-convex lenses with their plane sides next the 

 object, and placed at such a distance from each other as would best 

 correct the color. Though this microscope has advantages, I am sat- 

 isfied from experiments that the sextuple object-glass is decidedly 

 superior. 



Purple light may be obtained by causing the sun's rays to be re- 

 fracted through the prism, which will sufficiently separate the colors 

 when it is placed ten or twelve feet from the microscope. It 'is prob- 

 able that purple light would be preferable to any other, if it could be ob- 

 tained sufficiently strong ; but from experiment it appears that blue 

 light answers the purpose as well, on account of its greater intensity. 



