THE MICROSCOPE. 



41 



Instruments to perfection, and during the manufacture of the 

 object-glasses, he effected a most important improvement in 

 their construction, which he thus describes:* — "Having 

 applied Mr. Lister's principles with a degree of success never 

 anticipated, so perfect were the corrections given to the 

 achromatic object-glass, so completely were the errors of 

 sphericity and dispersion balanced or destroyed, that the cir- 

 cumstance of covering the object with a plate of the thinnest 

 glass or talc disturbed the corrections, if they had been 

 adapted to an uncovered object, and rendered an object-glass 

 which was perfect under one condition sensibly defective 

 under the other." This defect, if that be called a defect 

 which arose out of an improvement, he (Mr. Ross) first 

 detected, and immediately suggested the means of correcting, 

 and in 1837 communicated his discovery to the Society of 

 Arts, in a paper published in the fifty-first volume of their 

 Transactions, to which paper the author would refer those of 

 his readers who would wish to enter more fully into the 

 subject ; the desired object being effected by separating the 

 anterior lens in the combination from the other two; and 



figure 22, which is a section 

 of an achromatic object- 

 glass, will explain how the 

 principles established by 

 Mr. Ross were put into 

 practice. A represents a 

 tube, in the end of which 

 the anterior lens is set ; this 

 shdes on the cylinder, B, 

 containing the remainder of 

 the combination; the tube, 

 A, holding the lens nearest 

 the object, may then be 

 moved upon the cylinder, B, 

 for the purpose of varying 

 the distance, according to 

 the thickness of the glass covering the object, by turning the 



» Op. at. p. 8. 



Fig. 22. 



