64 SELECTION AND USE 



Corrected Objectives. The defects which we have just 

 described have been the chief difficulties in the way of perfect- 

 ing both the microscope and the telescope. In the case of the 

 latter, however, it was long ago found that very excellent re- 

 sults could be obtained by forming the lenses of two or more 

 pieces of glass of different kinds, and numerous attempts were 

 made to apply the same principles to the construction of the 

 microscope, but without marked success. The small lenses 

 used for the microscope seemed to defy the skill of the practi- 

 cal opticians of those days, and resort was had to such devices 

 as lenses made of precious stones, and the use of light which 

 could uot be decomposed mono-chromatic light as it was 

 called, or light of one color. Sach light is readily procured by 

 the combustion of alcohol mixed with common salt, and when 

 illuminated solely by such a light, a brilliantly colored painting 

 looks exactly like a plain black and white engraving. But 

 although the use of such a light lessens the evils caused by 

 chromatic aberration, they introduce another which is quite 

 serious objects which are really colored, appear in black and 

 white only. Moreover, such a light cannot easily be obtained 

 of a brilliancy sufficient to afford good illumination, and in 

 addition to this all the defects due to spherical aberration still 

 remain in full force. 



The first attempts made to perfect the object-glasses of micro- 

 scopes, consisted in the use of doublets and triplets, it having 

 been found that the spherical aberration is greatly lessened, 

 when the total refraction is divided up amongst several surfaces 

 of moderate curvature, instead of one surface in which the curva- 

 ture is excessive, and this plan is still pursued in the construc- 

 tion of what are known as French triplets, which will be de- 

 scribed hereafter. About the year 1829, however, Mr. J. J. 

 Lister, of London, England, published an elaborate paper 

 upon the subject, and it was from the principles laid down in 

 this paper, that all the important improvements in the modern 

 objective took their rise. These principles were embodied in 

 the practical construction of objectives by Andrew Ross, who 

 suggested the important improvement known as the adjust- 

 ment for thickness of cover. To Lister and Boss, therefore, 

 it may be justly said that we owe that optical wonder, the 



