1881.] 



MICROSCOPICAL JOURINAL. 



65 



Diaphragms. 



BY EDWARD PENNOCK. 



What is the use of diaphragms 

 beneath the stage ? It will, I think, 

 be admitted by all, that it is to pro- 

 perly regulate the illumination of the 

 field of the microscope, and to ex- 

 clude extraneous and confusing rays 

 which do not proceed from the mir- 

 ror. The only way in which the 

 diaphragm can affect the illumina- 

 tion of the field is by altering the 



size of the pen- ^ ^^ 



oil of rays pro- 

 jected by the 

 mirror.* It is 

 the purpose of 

 this brief note 

 to enquire how 

 thisobjectisbest 

 accomplished ; 

 whetherbysmall 

 diaphragms in 

 contact with the 

 object-slide, or 

 larger ones at a 



considerable 

 distance below. 

 The former plan 

 seems tobe com- 

 ing into favor 

 and has been 



adopted by 

 some of the best 



English and 

 American ma- 

 kers. 



Fig. 17 shows 

 the effect of a 

 diaphragm of 

 4"™" aperture, 

 placed at a dis- 

 tance below the slide. The pencil 

 of rays h g i oi an angular diameter 

 of about 14° is admitted to the center 

 of the field, and a like pencil h b i 

 to the margin, illuminating the whole 

 field evenly, supposing of course that 

 the source of light whose image is 

 formed at a b is of sufficient size and 



* It is assumed that the mirror is placed at 

 the distance of its focal length below the 

 object. 



Fig. 17. 



of uniform intensity, and that all the 

 rays are taken up by the object-glass. 

 The use of a ruler in connection with 

 the diagram will demonstrate the lat- 

 ter point (to avoid confusion the lines 

 are not given. in the engraving). With 

 an object-glass of i inchfocus, 36° aper- 

 ture, it is evident that the margin of 

 the field will be as brightly illuminated 

 as the centre ; with a lens of less 

 aperture the whole of the pencil illu- 

 minating the margin of the field, 

 however, will not be taken up. 



Suppose we obtain a pencil of 

 rays of the same angle, by means of 

 a small diaphragm placed near the 

 object, we will immediately find that 

 the field of view is very much cut 

 down, while the illumination of the 

 centre of the field is the same as be- 

 fore. In order to have the whole 

 field illuminated, it will be neces- 

 sary to use a diaphragm-aperture 

 as large as the field of view ; in which 

 case the diaphragm exerts no control 

 over the illuminating pencil, and we 

 have a flood of light from the mirror 

 and surrounding luminous sources, 

 destroying definition. A reference 

 to Figures 18 and 19 will make these 

 points clear.* 



Fig. 19. 



* In order to secure an evenly illuminated 

 field with very low-power objectives of small 

 angular aperture, it may be desirable to focus 

 the mirror upon the objective, thus transmit- 

 ting light through all parts of the object to 

 the object-glass. In this case, the position 

 of the diaphragm is not of so much impor- 

 tance, as it cannot then regulate the intensity 

 of the illumination to any great extent, even 

 when in its best position, but merely the size 

 of the field. 



