708 SUililAKY OF CUEEEN'T EESEAKCHES RELATING TO 



obtained, if tlie refracting surfaces of the same are equal in curvature, 

 and the ravs entering the lens are coming from a certain point of the 

 optical axis, being in distance from the lens a little over twice that 

 of its nominal focus, thus meeting at the other side at an equal 

 distance and forming a cone equal to that at the entering side. 

 Now there is a simple way to give the rays, coming from a distant 

 point or object, before entering the crown glass lens of the telescope, 

 a direction which will be nearly adequate to the fii'st-mentioned 

 condition, namely, if the flint glass lens is placed in front of the 

 crown glass lens. The parallel direction of the rays will then, by 

 the negative flint glass lens, be changed into such diverging direction 

 as would correspond with a cone, being only a little shorter than 

 that requii-ed for an equal-sided crown glass lens, and the latter 

 will then for minimum spherical aberration have to be very near 

 equal-sided, thus allowing the aberration of higher order to be in 

 higher degree reduced than in the before described objective. But, 

 however, as an objection to this arrangement, it may be mentioned 

 that the flint glass lens will be directly exposed to the external air 

 and liable to oxidation. 



" In my foregoing description I have, for the purpose of avoiding 

 complications and giving a clearer understanding, referred to the 

 telescope only ; but as the construction of this instrument is sub- 

 mitted to the same theoretical laws as that of the Microscope, little 

 remains to be said about the application of the described new prin- 

 ciple to the Microscope. Our present Microscope objectives are all 

 achromatic in the common sense, but they difter widely in angular 

 aperture, and accordingly in definition and resolving power. But the 

 angular aperture is dependent on the correction of the aberrations of 

 higher order ; the latter again on the spherical aberrations of the 

 crown glass lenses of the system. If the crown glass lenses are 

 transformed according to the described principle and law of minimum 

 spherical aberration, and then the flint glass lenses so changed as to 

 properly correct the aberrations of the crown glass lenses, the same 

 result will be obtained as with the telescope objective. The extension 

 of the lim it of angular aperture will admit of giving the low power 

 objective with long working distance a definition and quantity of 

 light which at present are united only in considerably higher po^vers 

 of short working distance." 



Care and Use of Objectives.* — Mr. "W. Wales uses only an old, soft, 

 silk handkerchief, a small stick of soft wood, a phial of adcohol, and a 

 watchmaker's glass of two powers. A camel's-hair brush can neither 

 completely nor safely remove the film of dust with which the exposed 

 fiurface of the back combination of an objective is sometimes found 

 to be coated. It will make a series of rings on the surface of the 

 lens, and it may, if grit be present, scratch the glass. Nor should 

 the handkerchief, either wet or diy, be introduced into the tube of 

 any but a low-power objective. The cells must first be unscrewed 

 from their mountings, and then the cleaning can be done properly. 



• Journ. N. York Micr. Soc, i. (18S5) pp. 113-6. 



