262 On the Achromatic Eye-pieces of Spy-glasses, 



pic objects, such as minerals, flowers, assemblages of the aquatic 

 larva*, &c. : and, accordingly, the double glasses of mine hav- 

 ing a combined focus of 2| inches, and the single one S\ inches, 

 form an admirable instrument of this sort, both separately and 

 in combination. Thus the single one in situ may be used by 

 itself by unscrewing and removing the double ones, which 

 may again be employed separately by removing the single one ; 

 lastly, the three may be used combined, either at a distance or 

 in contact (as in the eye-piece), giving a great variety of powers. 



There is an effect produced by shading object-glasses zvhen 

 of long foci and large aperture^ which I do not very well coni- 

 prehend. It is not observable in those of short foci. Thus the 

 single achromatic of 3J inches focus and | inch of aperture acts 

 most beautifully when in situ shaded by the projecting tube ^ but 

 if freely exposed, requires to be cut off to half an inch of aper- 

 ture to act as well. The same may be said of the two which are 

 combined in contact, whose performance is vastly improved by 

 a shade, — though it does not in any way reduce their aperture. 



When all the glasses are combined, the single one at a dis- 

 tance and acting negatively, the power being a minimum, if 

 a stop of half an inch of aperture is inserted immediately be- 

 hind the combined double achromatics, it will in no degree re- 

 duce the size of the visual pencil which enters the retina, but 

 greatly improve the effect of the combination. Whether it af- 

 fects the penetrating power of the object-glass or not, I have not 

 been able to ascertain as its focus is too long to shew lined test 

 objects. If this arrangement should be found innocent as re- 

 gards penetrating power, it ought certainly to be adopted in 

 object-glasses of moderate power. There is another lesson I 

 have learnt from the study of these large achromatics (which of 

 course give an immense visual pencil with a shallow eye-glass), 

 viz., that the iris does by no means act as a stop with regard to 

 optical instruments, or render inert and of no effect all that por- 

 tion of a visual pencil which exceeds its own diameter (for as 

 the visual pencil of a microscope or telescope is nothing but 

 the image of its object-glass or metal formed by the glasses 

 next the eye), at this rate the iris should be equivalent in effect 

 to a circlet placed over the objective end of the instrument, or 

 an eye-hole of its own aperture, which I am sure is not the 

 case. — June 9th 1838. 



