AMICI'S USE OF 'IMMERSION' LENSES 2/ 



who obtains distinct vision at 50 inches instead of 10 inches, and, in 

 fact, many microscopists declare the ordinary figures of amplification 

 to be useless for them because they cannot observe the image at the 

 supposed distance. It appears as if and many have this opinion 

 the performance of the microscope in regard to magnification 

 depended essentially on the accommodation of the observer's eye. 

 This misleading idea, resulting from the common expression, is 

 eliminated by defining the 10 inches merely as the distance from the 

 eye at which the image is measured whether it be a distinct or an 

 indistinct image. For, if an obsef ver, owing to the accommodation 

 of his eye, obtains a distinct image at a distance of 10 feet, I may 

 nevertheless assume a plane at a distance of 10 inches from the eye 

 on which the distant image is virtually projected, and measure the 

 diameter of that projection. Now this diameter is strictly the same 

 as the diameter of that image, which another observer would 

 really obtain with distinct vision at that same distance of 10 

 inches. 



The only difference is that in the former case we must take the 

 centres of the circles of indistinctness instead of the sharp image- 

 points in the latter case. If the conventional length of =10 inches 

 is interpreted in this way (as distance of projection, independently 

 of distinct vision) the absurdity at least of a real influence of the 

 accommodation on the power of a microscope is avoided. It becomes 

 obvious that for long-sighted and for short-sighted eyes the same N 

 must indicate the same visual angle of the enlarged objects, or the 

 same magnitude of the retinal image, because it indicates the same 

 diameter of the projection at 10 inches distance. 



It was long since pointed out by Amici, that the introduction of 

 a drop of water between the front surface of the objective, and 

 either the object itself or its covering glass, would diminish the loss 

 of light resulting from the passage of the rays from the object or its 

 covering glass into air, and then from air into the object-glass. 

 This, which is known as ' water immersion,' was, however, first sug- 

 gested by Sir D. Brewster in 1813. But it is obvious that when the 

 rays enter the object-glass from water instead of from air, both its 

 refractive and its dispersive action will be greatly changed, so as to 

 need an important constructive modification to suit the new condi- 

 tion. This modification seems never to have been successfully 

 effected by Amici himself; and his idea remained unfruitful until it 

 was taken up by Hartnack, who showed that the application of what 

 is now known as the immersion system to objectives of high power 

 and large aperture is attended with many advantages not otherwise 

 attainable. For, as already pointed out, the loss of light increases 

 with the obliquity of the incident rays ; so that when objectives of 

 very wide aperture are used ' dry,' the advantages of its increase are 

 in great degree nullified by the reflection of a large proportion of 

 the rays falling very obliquely upon the peripheral portion of the 

 front lens. When, on the other hand, rays of the same obliquity 

 enter the peripheral portion of the lens from water, the loss by re- 

 flection is greatly reduced, and the benefit derivable from the large 

 aperture is proportionately augmented. Again, the 'immersion 



