HARDWICKE'S SCIENCE-GOSSIP. 



251 



no longer brought to a nearly exact focus ; (2) that 

 the new focus, such as it is, is not, as before, in the 

 principal axis of the mirror, c D, but distant from it, 

 on the side away from the light ; (3) that the rays 

 reflected from the margin of the mirror are the most 

 unroly — that is, the most affected by what is termed 

 "spherical aberration ; " (4) that if, by means of the 

 diaphragm, we stop off all the reflected rays, except 

 those which come from near the central point D, the 

 latter rays will converge fairly well to a focus at a 

 point f' ; (5) that the distance of F'from D is less than 

 the principal focal distance, T> F, in Fig. 158. (In 

 Fig. 159 F° represents the position of F in Fig. 158, 

 and the reader can at a glance compare the two focal 

 lengths. It must be remembered, however, that the 

 figure does not represent the real mirror of the micro- 

 scope. If it did so, the difference of focus would be 

 much more considerable.) (6) that since the angle of 

 incidence of the ray at D is equal to the angle of re- 

 flection, it is equal to half the angle LD M, formed by 

 the same ray with the axis of the microscope, M D. 

 This relationship always exists between the two 

 angles, so long as the mirror remains in the axis 

 of the microscope, enabling us when one of the 

 angles is known, readily to find the other one from 

 it. In practice, it is convenient first to determine 

 the angle formed with the axis of the microscope, and 

 then by halving it to obtain the angle of incidence. 



Fig. 160 has special interest for us, because it repre- 

 sents a position of the light which is often adopted in 

 practice. Parallel rays here form an angle of 90 (a 

 right angle) with the axis of the microscope, and 

 therefore of 45 with the principal axis of the mirror. 

 The marginal rays have become increasingly unruly ; 

 but if we stop them off, the mean focus of the rest 

 will be about the point F 2 , which is nearer to D than 

 either F° or f'. The oblique marginal rays will 

 be usefully brought into service in certain cases (as 

 for the resolution of easy diatoms), by removing the 

 diaphragm, and bringing the mirror a little nearer to 

 the object than its mean focal length, so that the 

 object may coincide in position with the point F 3 

 instead of F 2 . 



In Fig. 161 the rays are represented as forming an 

 angle of 120 with MD the axis of the microscope. 

 The angle of incidence is therefore 6o°. The reader 

 will find, I think, that the construction of his instru- 

 ment will not permit the mirror to be brought much 

 closer to the object than this arrangement of the light 

 requires. In my own case it is necessary to slip the 

 mirror off its stem, and to reverse the clip. In most 

 cases 50 will be a more convenient angle of inci- 

 dence. 



By bringing the mirror a little nearer to the object 

 than the distance D F 3 , and dispensing with the 

 diaphragm, a good form of incident is obtained 

 for many " easy " diatoms. 



Thus far the incident pencil has been supposed 

 to consist of parallel rays, and it has been shown 



that, by merely varying their angle of incidence, both 

 the accuracy of the focus and its distance from the 

 mirror are entirely altered. We may suitably pause 

 here to enquire which of the combinations figured is 

 most to be recommended for ordinary use with trans- 

 parent objects. I think the rectangular position of 

 the light, as represented in Fig. 160, is the one 

 entitled to pre-eminence. The lamp is not so near 

 to the observer as to be in danger of being over- 

 turned, nor as to interfere with the free use of his left 

 hand in moving objects on the stage ; nor is it so far 

 from him as to throw light 'towards his eyes, and in 

 that way prove an annoyance. It is, moreover, the 

 only position of the light in which the angle of inci- 

 dence remains unaffected by alterations made in the 

 slope of the microscope, so long as the flame is kept 

 on a level with the central point of the mirror. This 

 will be explained presently. 



For special purposes other positions of the lamp 

 are preferable. The suitability for diatoms of the 

 arrangement in Fig. 161 has been already referred to. 

 For objects which require direct light, and for 

 brilliant dark-ground illumination, the more nearly 

 we can come to the use' of the " principal focus" the 

 better. For these cases the arrangement in Fig. 159 

 will be found very suitable. If the other angles be 

 adopted for direct-light objects, the diaphragm must 

 be interposed, and all but the central rays be stopped 

 off. In the next article, instead of passing on to the 

 subject of divergent rays, we will endeavour to use 

 practically the knowledge we have already gained. 



( To be continued. ) 



TEETH OF FLIES. 

 By W. H. Harris. 



No. X.— MUSCA MERIDIANA. 



IN the last paper but one of this series a very minute 

 set of dental organs was brought under review ; 

 in the present, the other extreme is sought, and by 

 far the largest and most numerous set of organs yet 

 met with is proposed to be dealt with. 



Musca meridiana is a large and rather handsome 

 fly, measuring a full half-inch in length, being pro- 

 portionately broad. At first sight it appears to be 

 quite black, but a close inspection discloses the fact 

 that it is of a very dark shade of blue, or what might 

 be more correctly termed blue-black. The basal 

 portions of the wings are of a deep amber shade, 

 gradually becoming quite clear towards the apex. 

 When its acquaintance is once made, it cannot be 

 easily mistaken for any other member of the Muscidae. 

 It frequents trunks of trees, palings, gates, and in 

 fact, delights in situations exposed to the direct rays 

 of the sun. It is generally reputed to put in an 

 appearance during the hottest hours of the day, and 



