MICROSCOPE. 



3,31 



required for making the slightest adjustment. 

 The axis of the body is never changed in the 

 least degree, which we have rarely found in 

 rack adjustments, owing to the pressure of the 

 pinion against one side of the bar ; and thus an 

 object is always kept in the centre of the field, 

 whatever change made be made in its focal 

 distance by an alteration of the magnifiers. By 

 a little practice, the power of making adjust- 

 ments of extreme minuteness may be obtained, 

 if the bar have been originally filed so true 

 that it works with perfect smoothness in every 

 part. An additional adjustment, which may 

 be made of any required fineness, is provided 

 for, however, by making the square tube or 

 socket itself moveable, and connecting it with 

 a micrometer screw worked by a large milled- 

 head at the bottom of the stem. By turning 

 this screw, the socket, and the bar which it 

 carries, are raised or depressed in any minute 

 degree ; and the socket being made to work 

 through stuffing - boxes closely packed with 

 cork, all twisting on its axis by the action of 

 the screw is avoided. On the fineness of the 

 screw the delicacy of the adjustment will of 

 course depend. That which we have employed 

 has forty turns to the inch; and the milled- 

 head being 1^ inch in diameter, or about 4 

 inches in circumference, a movement of one- 

 fiftieth of its periphery, or something less than 

 T ' 3 th of an inch, will affect the arm to the amount 

 of jjjggth of an inch. We should recommend, 

 however, a screw rather finer than this. By 

 dividing a circle on the milled head, and affix- 

 ing an index-point to the bottom of the stem, 

 the amount of motion given may be known to 

 a great nicety, and thus the thickness of a mi- 

 nute object laid upon any surface may be mea- 

 sured. If its upper side be brought exactly 

 into focus, and it be then removed, and the 

 surface on which it has lain be brought into 

 focus by the micrometer screw alone, the num- 

 ber of divisions over which the index has passed 

 will of course indicate the thickness. This me- 

 thod, which was proposed by Mr. Valentine,* 

 answers very well for such objects as the vessels 

 of plants, scales of insects, &c. which can be 

 completely isolated, when a sufficiently high 

 power is employed, so that distinctness can 

 only be obtained at one point. We consider it 

 a great advantage of this kind of adjustment, 

 that it is effected at a considerable distance 

 from the stage, and that the hand is therefore 

 in no danger of deranging the position of ob- 

 jects or apparatus connected with it. 



The arm which carries the body is attached 

 to the top of the bar by a screw-pin passing 

 through the former, by which it is enabled to 

 traverse from side to side. This movement 

 will often be found extremely useful, both in 

 the examination of different parts of objects 

 which it is desirable not to move, and in chan- 

 ging magnifiers, &c. on the body ; the want of 

 it we consider one of the chief inconveniences 

 in Mr. Ross's form of construction. The arm 

 is ths of an inch thick, and broad enough at 

 the part farthest from the centre to receive the 



* Trans, of Soc. of Arts. 



body, which is not made tapering in the usual 

 manner, but is attached by a screw of 1 inch 

 diameter, with a large shoulder above it. By 

 this mode of attachment, and by the massive- 

 ness of the arm itself, all vibration from the top 

 of the bar is prevented ; at least ive have not 

 been inconvenienced by it. When the bar is 

 pushed into its socket, so that the arm ap- 

 proaches the stage, (as when high powers are 

 used,) no oscillation can arise from its vibra- 

 tions ; and the socket itself works through an 

 aperture in the stage-plate, to which it is so 

 closely fitted that no oscillation can arise in 

 that point; so that, both in theory and prac- 

 tice, we find the form here proposed unobjec- 

 tionable on this score. It is true that, when 

 the bar is drawn out to its full extent, oscilla- 

 tion may arise ; but this is never the case, ex- 

 cept when low powers are being employed, and 

 then we altogether fail to perceive it. In the 

 construction of the body there is nothing worthy 

 of peculiar remark ; and, as we have already de- 

 scribed the various modes of magnifying the 

 object, we shall therefore now pass on to con- 

 sider the best means of illuminating it. 



The perfect illumination of the object is a 

 matter of the utmost importance, especially 

 when high magnifying powers are being em- 

 ploj ed. There are many difficult objects which 

 require to be viewed under a great variety of 

 aspects, in order that their true characters may 

 be determined ; and there are not a few whose 

 structure cannot be understood at all, even with 

 the most perfect arrangement of the optical 

 portion of the microscope, unless similar atten- 

 tion be bestowed on the concentration of the 

 light by which they are viewed. We shall, 

 therefore, bestow on this subject more attention 

 than it has ordinarily received. 



For transparent objects of large size, which 

 are being viewed with low powers, such as sec- 

 tions of wood, wings of insects, Sec. we find a 

 concave mirror by far the most simple and, at 

 the same time, effective means of illumination ; 

 and the optical errors to which it gives rise are 

 not such as to interpose any practical difficulty 

 in its use. It should be, for such a microscope 

 as we have described, of greater size than that 

 ordinarily employed ; three inches may be re- 

 garded as a good diameter. It should be set, 

 by an universal joint, upon a piece of tube 

 fitted to slide stiffly up and down the stem 

 which descends from the stage. In this man- 

 ner its distance from the object may be readily 

 varied ; and the degree of concentration of light 

 effected by it will thus be easily adapted to the 

 character of the object to be viewed. Thus, if 

 it be pushed near the stage, the pencil of con- 

 vergent rays will not be nearly brought to a 

 focus, and a large surface will be illuminated 

 with a moderate light. But if it be drawn 

 nearer the opposite end of its range, the rays 

 will be more concentrated, so that a smaller 

 surface will be illuminated, but with much 

 greater brightness. By the former adaptation 

 we are enabled to illuminate, with great equa- 

 lity, and by means of the ordinary lamp-flame, 

 an area of three-eighths of an inch in diameter, 

 with sufficient intensity to produce a very bright 



