62 



THE MICROSCOPE. 



tant factor in the production of the image. An object 

 viewed without a cover-glass is more clearly defined 

 than one with. The late Andrew Ross explained the 

 cause of this difference in a paper published in the 

 " Transactions of the Society of Arts," vol. 41. 



Perfect definition is the quality most sought after by 

 those engaged in histological pursuits ; whilst perfect 

 resolution is more highly esteemed by those who take 

 especial interest in the finest diatoms and test objects 

 of a similar nature. Various modifications have taken 

 place in the construction of the object-glass of the 



FIG. 37. Forms of Object-glasses. 



A, Double-convex lens ; B, Plano-concave ; C, Bi-convex and plano-concave 

 united ; shown in their various combinations, as at D, form the 3-in., 

 2-in. or IJ-in. ; at E, 1-in. and jj-in. ; and at F, the -in. -in. J-in. and 

 5 i 3 -in. objectives. 



microscope : opticians, however, are quite agreed that 

 the highest theoretical perfection will be obtained by 

 an increase rather than a decrease in the number of 

 lenses entering into its combination. Both at home 

 and abroad first-class makers, such as Ross, Powell 

 and Lealand, Beck, Dallmeyer, Tolles, Wales, Zeiss, 

 &c., have been working on this principle. To a well- 

 considered combination formula they have added a 

 single front plano-convex lens of crown-glass, which 

 gives increase of power with a longer working distance 

 to the objective. 



