BY MEANS OF THE MICROSCOPE. 389 



time to prepare more than one or two. One of the advantages of 

 their use is that, if the object is very dehcate and likely to be 

 flooded with light, the use of the screen comes in, and for objects 

 that it would be almost impossible to expose quick enough ; the 

 insertion of a suitable screen enables you to make a good expo- 

 sure and get all detail, notably in the dehcate membrane of some 

 of the wings of flies, etc. 



With regard to illumination, as I have said before, an oil lamp 

 will do very well ; but if you can get the limelight it is more 

 satisfactory. The light is more pure in colour, more intense, and 

 more easily under control. 



Working from a small spot of light, and the smaller and more 

 intense the better, the ray goes more direct through the centre of 

 the optical system than a large volume scattered about. A blow- 

 through jet will answer all purposes, and if it is fitted with a 

 Pringle cut-ofl" — that is, an arrangement for lowering the gases 

 between the exposure — you save the gas and also make sure of 

 getting the same amount of light each time. 



For time of exposure one can give no fixed rule, so much 

 depends on the subject, its colour, thickness, the amount of den- 

 sity, and the magnification wanted. With reference to the plate, 

 use a slow, thickly-coated one for most work. The isochromatic 

 will be found very useful for many subjects, but I have not found all 

 the advantages that are claimed for them, for of the number of 

 slides that I hope to show you very few are taken on those plates. 

 Nearly all my negatives were done on Paget xxx or xxxxx. 



Each class of object requires a special study. By class I mean 

 transparent, high or low power, opaque, objects taken on a dark 

 ground, and objects taken by means of polarised light ; each of 

 these I shall more or less speak upon. The ordinary objects, 

 such as a blow-fly's tongue, wing of bee, stings, head, internal and 

 external organs, sections of scalp, and thousands of others, can be 

 taken in the way I have explained, and with a 4-ioth to 2 in. 

 objective with very little trouble and not much practice. High- 

 power work, using an immersion lens of, say, i-i2th, requires a 

 very great deal of care and manipulation of the whole apparatus, 

 from the achromatic condenser to the ocular. 



Many objects that are opaque are very interesting to photo- 



