16 PHOTO-MICROGEAPHY 



distance is greate7' than its focus : this causes the rays to come to a focus say at F. 

 Certain class of work demands this position in preference to the other. It will be 

 seen now how the condenser is used, not only to grasp light, but to direct the rays m 

 given directions. A large one is best because its central rays will only be employed.* 

 To resume. Owing to the intense heat to which the glasses of the condenser, 

 especially that one nearest the light, are subjected, it has been proposed, and used by 

 many, that an additional protective glass plate should be interposed between the con- 

 denser and the limelight, but we have found this to be unnecessary, especially when 

 using the ground glass in that situation, for that in itself affords a protection to the 

 posterior lens, and the less the amount of glass interposed between the light and the 

 object the stronger will be the resulting illumination. 



One more point of practical importance must not be overlooked, and that is, the 

 photographer should carefully examine the condenser before using it to see that the 

 lenses^'are quite loose in their cells, for if they are in the least measure tight when 

 cold, the expansion which takes place on their becoming heated will surely cause one 



or both of them to crack. 



Having explained the position and details of the light, its box, and the condenser, 

 we will pass on to consider the support (marked S in the diagram) for the object we 

 propose to photograph. Let this be, for example, a moderately -sized spider, and that 

 we want to enlarge it to fill a lantern slide. These are procurable laid out on an 

 ordinary microscopical slip 3 by i inches. It is placed upon the support S (which, it 

 is seen in the diagra.m, is merely an upright piece of wood with a hole m it), being 

 held there in position by two ordinary clips, such as are used in the microscope. 

 They consist of two pieces of spring, each being mounted on a brass pm. The support 

 is seen to be fixed at right angles to another piece of wood, which is nothing more 

 than the base board of a camera, capable of being racked to and fro by the milled 

 head shown beneath. As will be explained hereafter, this is for the means of final 

 focussing. 



We next come to a difficult part of our subject, namely, the selection of a lens. 

 It has been already stated that the microscope affords too limited a field of view for 

 our requirements, a remark which also applies to the use of microscopical objectives. 

 It IS easy to understand this, for the field of view for which that class of objective is 

 corrected is naturally small, being only intended to be used with the microscope 

 The consequence of this is that if a microscopical objective be placed on the camera at 



* This will be better understood when the subject is treated ot in its entirety further on, 



