SCIENCE-GOSSIP. 



[Jan. 1, 1865. 



artificial eye witli a focus considerably shorter 

 than his own. He makes use of it by placing 

 his own living eye behind it, and looking 

 through it at that which is beyond it. His 

 own eye is thus brought much closer to the 

 object than it could be without the aid of his 

 lens — the result being an enormous accession 

 of enlarging power. Be it observed, how- 

 ever, that the lens does not augment the size ; 

 it only enables the eye to be brought nearer, 

 for could the eye see at a similar short distance 

 without the help of the lens, the result would 

 be precisely the same as that attained by the 

 use of the lens. The real service that our 

 piece of convex glass renders to us is this — 

 it takes the divergent rays which emanate 

 at all points from an object situated nearer to 

 the eye than its natural focus, and brings 

 thetn into a proper condition to be received 

 by the eye, which is then competent to deal 

 with them in forming a perfect image on its 

 retina. Without the lens this could not be 

 accomplished : the image would then not fall 

 on the retina, but at some distance beyond it. 

 Let us now apply this principle to the opti- 

 cal part of the compound microscope. At each 

 end of a brass tube some 10 inches in length 

 and 1^ inch in diameter ai'e placed the lenses 

 which produce such marvellous results. At 

 one end of the tube nearest to the eye of the 

 observer is placed the eye-piece, at the oppo- 

 site end is the object-glass, so called from its 

 proximity to the object. We will first deal 

 with the latter : it may be a lens or combina- 

 tion of lenses of high or low magnifying 

 power. By the mechanical aid of rackwork 

 and pinion we contrive to bring the lens into 

 focus with a small object placed on the stage 

 of the microscope. The light transmitted 

 or reflected from this object enters the 

 object-glass and at some distance in the 

 interior of the tube produces a faithful 

 copy of the object greatly increased in 

 size. For the sake of illustration we will 

 consider the copy, or image, thirty times 

 loager, broader, and deeper than the original. 

 Now, although the eye is not present at the 

 spot where this image is formed to receive it, 

 yet we very well know that it is actually 

 there, and placed, too, in a most convenient 

 ]icsition to be seen by looking at it through 

 tlie eye-piece. If the latter had the power of 

 doubling the size of the first image, the result 

 would be a second image twice the size of the 

 first, and sixty times larger in length, breadth, 

 and depth than the object itself And this 

 is Avhat really takes place in the compound 

 microscope. We have, as it were, two simple 

 nncroscopes. The first deals with the object, 



and the second with the enlarged image that 

 object forms. It is in consequence of this 

 compound ai'raugement that the modern 

 microscope has received its present name. 



It is greatly superior to its predecessoi*, the 

 simple microscope, which labours under the 

 disadvantages of being more limited in its 

 range, the result of producing but one image, 

 and the necessity forced on the observer to 

 have both eye and lens almost in contact with 

 the object he may wish to investigate. The 

 compound microscope, while it preserves us 

 from these drawbacks, secures for us a double 

 advantage. Its magnifying power is much 

 greater in consequence of its ability to pro- 

 duce tv/o images instead of one. And this is 

 secured to us, while the eye remains at the 

 ordinary distance of ten inches from an 

 object. 



We have endeavoured to show in a simple 

 manner, without going inf'^o optical techni- 

 calities, why objects appear so greatly enlarged 

 when seen under the Microscope, and that 

 magnitude, or increase of size, depends on the 

 distance of an object from the eye — that the 

 closer the eye can approach to it and see it, 

 the larger it will appear, and that the lenses 

 employed in the construction of our Micro- 

 scopes are only helps to bring about such a 

 result, T. K. 



A DEAD FLY ON THE Y7IND0W. 



^THO will prescribe for an unhappy fly ? 

 V, We were led to ask this question in 

 all simplicity the other day, by seeing one of 

 these disturbers of our usual afternoon nap 

 apparently in the agonies of approaching 

 death. As we regarded his rotund and in- 

 flated body, vague notions of the veterinary 

 art and even of Mr. Banting flitted through 

 our mind, but help seemed beyond the in- 

 sectile reach. Its beautiful aiTangement of 

 feet to climb glass and walk in an inverted 

 position over our heads, its delicate wings of 

 prismatic hues, its minute spiracles so con- 

 structed for breathing as to render a nose a 

 useless appendage, and its numerous eyes 

 Y.'hich enable it with equal readiness to see a 

 dainty morsel or a sinister enemy, all were 

 about to cease their functions, for disease had 

 arrested its playful gambols and thieving 

 propensities ; the sugar basin would " know 

 it no more," and our afternoon nap v/ould be 

 disturbed by one intruder the less. 



Still the death of a fly under these circum- 

 stances is not without its interest to those 



