498 SUMMARY OF CUERENT RESEARCHES RELATING TO 



as any other for ordinary observations, and for this purpose we have 

 added an adapting piece by means of which the usual eye-pieces may 

 be used. It is not difficult to understand the motives which have 

 led the makers to construct narrow tubes in Microscopes designed 

 for ordinary work ; the dimensions of the tube are determined by 

 those of the eye-piece, which, in order that the observer may not be 

 fatigued, should only receive so much of the image as may be con- 

 veniently comprehended by the eye." 



With regard to the special difficulties presented by objects which 

 are not flat, the author writes, " We have already stated that to 

 obtain a photograph well defined in every part, the object should be 

 as nearly as possible a plane. Unfortunately, even in the case of 

 sections, this condition is not always realized, while with certain 

 objects, e. g. insects and Foraminifera, it can never be so. It is pos- 

 sible, however, by certain methods to obtain perfectly clear photo- 

 graphs of objects which lie in different planes. When such a case 

 presents itself, it is well to use the weakest possible objective which 

 will bring out the details that are to be reproduced. By employing 

 a weak objective with long focus, many more planes can be simulta- 

 neously brought to a focus than with a more powerful one. The 

 desired result may also be obtained by stopping the objective with 

 a diaphragm ; the smaller the diaphragm the greater will be the 

 depth of focus, but at the same time the definition of the lens will 

 be proportionately diminished. A happy mean must be preserved in 

 the choice of a diaphragm. 



If the above means are not sufficient, we must have recourse to the 

 method of successive exposures. This method is based upon the fact 

 that the same sensitive plate may receive two or more images without 

 confusion ; this may be shown as follows : — Place on the stage a 

 micrometer, bring its divisions to a focus on the ground glass, then 

 insert the sensitive plate and expose for say two minutes. Intercept 

 the light, rotate the micrometer through an angle, and expose again 

 for two minutes. The plate when developed will show two crossed 

 images of the micrometer which are perfectly clear even at the point 

 where they intersect. In this way, three or even four superposed 

 images may be obtained upon the same plate. From the observation 

 of these facts, I was led to use the method of successive exposures in 

 the case of objects which could not be simultaneously focused in all 

 their parts. If the same plate receives in succession the images of 

 the different planes of an object, these will be superposed without 

 confusion, and a compound image will be produced which is far more 

 complete than that obtained by photographing a single plane. 



In employing this method, the head of the micrometer screw 

 should be provided with an index which moves upon a graduated 

 circle (fig. 84). The lowest part of the object being first brought to 

 a focus upon the ground glass, the division at which the index stands 

 is noted, then the highest part of the object is focused and a second 

 reading is made on the circle. These readings determine the limits 

 between which the index must move if all the successive planes of the 



