356 



MICROSCOPE. 



upon the object ; and, with a little practice, 

 a very close estimate may be formed of the 

 proportional size of the object, when it only 

 extends over a part of a single division. This, 

 for ordinary purposes, is by far the most con- 

 venient mode of measurement. 



The camera lucida, however, which has been 

 adapted to the microscope for the purpose of 

 delineating representations of microscopic ob- 

 jects, may be most advantageously applied 

 also to micrometry. By this instrument (of 

 the construction of which we shall presently 

 speak) a highly magnified picture is projected 

 upon a surface on which its outlines may be 

 easily marked, and on which their size may, 

 therefore, be determined with the greatest 

 nicety. Here, as in former cases, the micro- 

 meter object-glass must first be employed, in 

 order to fix the standard. If one of these be 

 placed in the focus of the microscope, and the 

 camera lucida be so adjusted, that an image of 

 its lines be thrown upon a piece of paper at a 

 fixed distance from it, the distance of these 

 may be marked with precision ; and subdivi- 

 sion on the paper may be carried to any re- 

 quired extent, so as to afford the means of at 

 once ascertaining the size of an object placed 

 in the field. Thus, if the magnifying power 

 and the distance of the paper be so adjusted, 

 that the lines which are really -jigth of an 

 inch apart are projected upon it at five inches 

 distance from each other, every inch on the 

 paper will of course be equivalent to 



of an inch on the object. Lines of ^jth of 

 an inch apart may easily be drawn on the 

 paper ; and the distance between each of these 

 will represent tu^jth of an inch on the ob- 

 ject. In this manner the size of an object may 

 be known with great nicety, and with less lia- 

 bility to error than in the use of the screw mi- 

 crometer. It is easy to increase the apparent 

 size of the image thrown by the camera lucida 

 to almost any required extent ; so that even 

 greater minuteness may be attained. The dis- 

 tance between the eye-piece and the paper may 

 be increased, either vertically by placing the 

 latter upon a chair or even on the floor, or 

 horizontally, by turning the prism or mirror 

 a quarter round, and projecting the image in 

 the direction of the side of the room, so that 

 the range of distance is much increased. Such 

 a plan is, of course, of no use in delineation ; 

 but in micrometry it may be had recourse to 

 with advantage, especially when comparing the 

 relative sizes of similar objects, such as the 

 blood-discs. For every magnifying power, 

 whether gained by changing the objective or by 

 increasing the distance of the screen, a determi- 

 nate value must of course be ascertained for 

 the divisions of the latter. 



The camera lucida of Dr. Wollaston is some- 

 times applied to the eye-piece of the micro- 

 scope for the purpose of delineation and micro- 

 metry ; but it is much inferior for these pur- 

 poses to other plans which have been devised. 

 Probably the best of these consists of a mirror 

 composed of a thin piece of rather dark-coloured 

 glass cemented on a piece of plate-glass, in- 



clined at an angle of 45 in front of the eye- 

 glass. Of the light which passes out from the 

 latter, a sufficient quantity is reflected by the 

 mirror to give a distinct image; and yet the 

 paper and pencil can be distinctly seen through 

 the glass, though rather darkened by the co- 

 loured glass, which thus serves to render the 

 image more brilliant. A lens is placed below 

 the reflector, which causes the rays from the 

 paper and pencil to diverge at the same angle 

 with those received from the eye-glass ; so that 

 both the object and the pencil are seen with 

 equal distinctness. The use of a small highly- 

 polished steel mirror, fixed in the focus of the 

 eye-piece, and inclined upwards towards the 

 eye at an angle of 45, is by some preferred to 

 this. The mirror being smaller than the pupil 

 allows the rays from the paper to pass up into 

 the eye around it ; and thus the image is seen 

 as upon the screw. In the use of either of 

 these instruments, the chief difficulty (as in the 

 use of the common camera lucida) is for the 

 delineator to see both the image and pencil 

 with sufficient distinctness to enable him to 

 make an accurate tracing of the former. Much 

 will depend upon the advantageous adjustment 

 of the amount of light upon the object and the 

 paper respectively. In drawing or measuring 

 by lamp-light, we have found it useful to place 

 a small taper near the screen, so that its direct 

 rays may fall upon it, whilst the lamp is used 

 for illuminating the object ; and when the 

 screen is illuminated by daylight it is prefer- 

 able still to use the lamp for the other purpose. 

 The point of the pencil should be blackened. 

 The micrometer eye-piece also may be em- 

 ployed for drawing ; its squares being repre- 

 sented by squares on the paper ; and the por- 

 tion of the object between each being delineated, 

 in the manner commonly practised by artists. 

 No assistance of this kind, however, can supply 

 that skill to the microscopic draughtsman which 

 is required for making finished delineations of 

 any object. Accuracy of outline is all that 

 they can ensure. 



Under this head it seems not inappropriate 

 to introduce a few remarks on the degree of 

 minuteness in the structure of objects, which 

 the magnifying power of the microscope ena- 

 bles us to detect. 



Much speculation has taken place amongst 

 philosophers at different times, relative to the 

 possibility of detecting the ultimate atoms of 

 material, especially organic, substances ; and 

 microscopists have occasionally hazarded state- 

 ments in regard to their size, which an in- 

 creased knowledge has shown to be invalid. 

 It was a favourite theory about fifteen years 

 since, that all organized bodies are made up of 

 globules, which cannot be resolved into any 

 other kind of structure, the diameter of which 

 was stated at about g^th of an inch. The 

 great improvements which have been recently 

 made, however, in the microscope, and the 

 general advance of knowledge on the subject 

 of the ultimate constitution of organized struc- 

 tures, have shown the erroneous nature of this 

 view, by proving that there is no body, however 



