1882.] 



MICROSCOPICAL JOURNAL. 



73 



Let us suppose that the ilhimination 

 is received from the left of the ob- 

 server, and that a micrometer is in- 

 serted in the eye-piece to facilitate 

 observation. Take three points, A, 

 B and C, in the optical axis, A, be- 

 yond the focus of the objective, B 

 at the focus, and C a little above the 

 focal plane. Suppose a pencil sent 

 from the mirror along the axis, passing 

 A, B and C, and the centre of the 

 objective. The images of A, B and 

 C will fall with their centres on the 

 axis. If the edge of the mirror to- 

 ward the observer's right be tilted up, 

 the point A, beyond the focus, will 

 appear to be displaced toward the 

 right of the field of view, the point B 

 will remain stationary, and C, which 

 is above the plane focussed upon, will 

 move toward the left. Now it can 

 be shown that if the spherical aber- 

 ration of an objective could be cor- 

 rected for a series of points and their 

 images, all the images must remain 

 stationary. 



The necessity for correction con- 

 sists essentially in the fact that the 

 margins of lenses with spherical sur- 

 faces are too strong relatively to their 

 centres. Hence, with an uncorrected 

 lens, the image of the point B, made 

 by the central portion of the lens, 

 would fall on the axis ; but an image 

 of the same point, produced by rays 

 entering the left-hand margin, would 

 fall to the left of the axis, as well as 

 nearer to the lens. The essence of cor- 

 rection is to relatively weaken the 

 action of the margin, so that the im- 

 age shall fall on the axis, and at the 

 same distance from the lens as the 

 image formed by its central portion. 



Suppose this correction to be made 

 for the point B. Let the mirror be 

 tilted as described, so that a pencil 

 of rays passes through A to the left 

 margin of the objective. This pencil 

 makes a smaller angle with the front 

 surface than a pencil coming from B 

 and entering at the same place. 

 Hence, the pencil from A will reach 

 the back surface of the lens at a point 

 nearer the axis than would be reached 



by the pencil from B. If then the 

 pencil from B comes to a focus on 

 the axis, the pencil from A would 

 cross the axis before coming to a fo- 

 cus. This explains the displacement 

 of the image a to the right, under the 

 conditions given above. The image b 

 of the point B will not be affected, be- 

 cause the objective is corrected for a 

 cone of rays from B, and any pencil 

 passing through B must coincide with 

 some element of the cone. It is not 

 necessary to discuss the image <:, for 

 it will be seen that it must be formed 

 on the side of the axis opposite to a. 

 Rutgers College. 



On a Convenient Method of Im- 

 bedding. 



BY PROF. E. A. BIRGE. 



The following method of imbedding 

 was worked out by Dr. Justus Gaule, 

 of the Physiological Institute, Leipzig, 

 Saxony, by whom it was communi- 

 cated to the writer. I have tried it 

 on all sorts of tissue and can fully 

 recommend it. 



A piece of tissue of convenient size 

 is to be taken, treated with the ordi- 

 nary reagents and stained in the mass. 

 If large, it may be convenient to re- 

 move it from the staining fluid to al- 

 cohol for a few hours and then re- 

 place it. When thoroughly stained, 

 the specimen is to be put in seventy 

 per cent, alcohol for about twelve 

 hours, then transferred to absolute 

 alcohol until it is completely dehydra- 

 ted. Then put it in oil of cloves over 

 night, or leave it there until it is con- 

 venient to imbed it. 



Place it in turpentine half an hour, 

 — large specimens for a longer time — 

 then transfer it to a mixture of tur- 

 pentine and parafifine, kept melted on 

 a water-bath at about 40° C. In this 

 the specimen, if from liver or intestine, 

 etc., should remain for an hour or 

 more; small nerves and blood-vessels 

 of course need not remain so long. 

 Then transfer it to a bath of pure 

 paraffine, melted at a temperature 



