ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



107 



however, decide how far it is advisable to add to Microscopes focussing 

 at the nose-piece, extra appliances tending to aflfect the delicate fitting 

 of the fine adjustment. 



Fig. 1G. 



Fig. 17. 



Fig. 18. 



Deby's Screw-Collar Adjustment. — Mr. J. Deby suggests that the 

 application of a worm-wheel and tangent screw to the screw-collar 

 adjustment of objectives (Fig. 18) would 

 be found more convenient than the usual 

 system for adjusting the corrections with 

 accuracy. The device, as figured, would 

 not permit the objective to be enclosed 

 in the ordinary brass box ; but, as sug- 

 gested by Mr. Beck, the tangent pinion 

 might be cut off short and provided with 

 a slightly tapering square head upon 

 which the milled head would fit when 

 required. 



Number of Lenses required in 

 Achromatic Objectives. * — Mr. W. 



Harkness discusses the number of 

 lenses required in an achromatic objec- 

 tive consisting of infinitely thin lenses 

 in contact, in order that with any given 

 law of dispersion whatever, the greatest possible number of light-rays 

 of different degrees of refrangibility may be brought to a common 

 focus. 



For any system of thin lenses in contact we have 



^ = (Ml - 1) Ai + (j,, - 1) A, -f 0"3 - 1) A3 + etc., (1) 



the number of terms being unlimited. For a dispersion formula we 

 write 



/^ = 'PM (2) 



The form of (X) is unknown, but there will be no loss of gene- 



* Bull. Phil. Soc. Washington, iii. (1878-80) pp. 65-7. Smithsonian Misc. 

 Collections, xx. (1881). 



