SCIENCE-GOSSIP. 



277 



objective other conditions being, of course, equal 

 — that enables us to perceive the tine structure in 

 an object under examination, and every degree of 

 aperture represents the ability of a lens to divide so 

 many lines to the inch. It, therefore, only becomes 

 necessary to be aware of the greatest aperture that 

 has been obtained, and the limit of vision possessed 

 by a keen eye, to know what is the maximum 

 magnifying power necessary to show all that the 

 microscope can reveal. The greatest numerical 

 aperture hitherto obtained has been 163, in the 

 T Vinch mono. brom. of naphthalin immersion 

 objective, made by Zeiss, which requires special 

 cover glasses and other exceptional conditions for 

 satisfactory working. In practice, however, 14 

 numerical aperture is the limit of aperture em- 

 ployed. 



The ability of the eye to separate detail has been 

 dealt with 'by Mr. E. M. Nelson, in an admirable 

 paper read by him before the Royal Microscopical 

 Society (" Royal Microscopical Society's Journal," 

 February, 1S93). In it he pointed out that a 

 keen eye could perceive all the detail that a 

 numerical aperture of 026 could resolve, if the 

 magnification were 100 diameters. That is, given 

 an eye-piece power of ten diameters, o 26 

 numerical aperture could be 

 given to every ten diameters 

 of initial magnification afforded 

 by the objective ; the com- 

 bined power of the eye-piece 

 and the objective producing the 

 needed 100 diameters. (The 

 initial magnification of an 

 objective can be very approxi- 

 mately determined by dividing 

 its focal length into ten. Thus 

 a |-inch would have an initial 

 power of seventy diameters.) 

 A further enlargement of the 

 image could be obtained, if 

 desired, by using eyepieces of 

 higher power. Bearing in 

 mind these two points, the 

 highest aperture obtained and 

 the limit of vision, it will be 

 seen that a ^-inch of 163 

 numerical aperture would more 

 than suffice, if used with an 



eyepiece power of ten, to show all the fine detail 

 that the microscope can reveal. 



The yV.-inch before referred to, so far as the writer 

 has been able to ascertain, was a dry objective. 

 The maximum numerical aperture obtainable with 

 a dry lens is 10; this lens, therefore, could not 

 have exceeded this. From the premises before 

 stated, it will be apparent that a J -inch objective of 

 similar aperture, used with an eyepiece, magnifying 

 ten diameters, would show all the delicate structure 

 that this ^-inch would be capable of doing, and 

 there is no doubt, whatever, that a modern 

 apochromatic objective, such as that made by Zeiss, 

 with its improved corrections for colour and 

 spherical aberration, would reveal even more than 

 the ^-inch lens. How absurd, therefore, is it to use 

 dry ienses of high power. 



In mentioning the limit of power to aperture 

 that is permissible, it must not be taken for granted 

 that it can be arbitrarily carried out. There are 

 very few lenses that have the full relation of 

 aperture to power. A saving in cost of manu- 

 facture is generally effected by having the power 

 fairly high for the aperture employed, and the 

 j^-inch has become the popular high-power 



objective of large aperture. This is not an 

 unreasonable power, but it is obvious that no 

 advantage can be gained by having a ,',,-inch, 

 unless, indeed, a proportionate increase of aperture 

 be given to it, which, so far, has proved to be 

 impracticable. 



Professor Abbe has been said to have j,'iven it as 

 his opinion that the £-inch immersion lens of 

 1*4 numerical aperture will be the lens of the 

 future, and the opinion of microscopists is becoming 

 increasingly favourable to this idea. 



Only those who have had experience in working 

 with the old high-power lenses and the modern 

 comparatively low powers of large aperture 

 can be aware of the advantages that the latter 

 possess. Not only are the latter far easier to 

 use, and yield a much brighter and more perfect 

 image, but they have a convenient working dis- 

 tance, which was a quality almost unknown in the 

 former. 



To summarize : attention should be paid by 

 microscopists in the selection of their objectives, 

 to see that they have at least a fair ratio of aperture 

 to power, and when it is thoroughly understood 

 that high magnification, unaccompanied by pro- 

 portionate aperture, is valueless, useless high- 



ADJUSTABLE OBJECT-HOLDER. 



power lenses will cease to be. — M. J. Cross (author 

 Part I. "Modem Microscopy"), 27, Chancery Lane, 

 London, W.C. 



Adjunct to Microtome. — In connection with the 

 new rocking microtome, described onpage22S(irW<'), 

 the Cambridge Scientific Instrument Company have 

 brought out a new form of adjustable object-holder, 

 sometimes called an " Orientating Apparatus " ; 

 this allows the object to be moved so that the 

 sections are cut in the required plane. The en- 

 graving shows this addition as applied to the old 

 rocking microtome, but a larger form is made suit- 

 able for the new instrument. It will be seen that 

 it consists of a ball-and-socket-joint which is 

 capable of being adjusted in all directions by 

 screws. The ball of the ball-and-socket-joint is 

 really a cup holding the paraffin in which the object 

 is embedded, and when both the screws (a) are 

 tightened this cup is pressed tightly into the socket, 

 and the whole is perfectly rigid, a point of consider- 

 able importance. The screw (b) moves the object 

 about a horizontal axis and the screws (a) move it 

 about a vertical axis. When the screw (c) is loosened 

 the whole apparatus can be at once removed 



