Heliostat in Micro-phoiograiohy. By G. M. Giles. 29 



matt varnish, is better. The instrument may be used either in its 

 simple or its compound form ; in other words, with or without the 

 eye-piece. 



When the eye-piece is not used, the camera must be drawn out 

 to a great length to obtain sufficient amplification, whereas at 10 

 inches distance from the eye-piece one gets the full nominal power 

 of the instrument. 



On the whole, I prefer to use the eye-piece, though seldom at 

 the full 10 inches. 



Almost any good objective will give good results, but micro- 

 photography is a very trying test to a glass. 



A very essential quality is flatness of field ; should this be want- 

 ing, its absence will be painfully conspicuous in every impression 

 taken. When the sun shines clearly, the most preferable process is 

 the ordinary wet collodion, on account of its handiness and easiness ; 

 but wet plates will not bear being kept for more than five or ten 

 minutes before exposure, so that they are very inconvenient to use 

 in cloudy weather when the sun shines out only occasionally be- 

 tween the clouds. On such occasions it is better to use dry plates, 

 in the form of the gelatine-bromide process. They have the ad- 

 vantage too, that one may develop them at one's leisure, but require 

 care in exposure, as they are more sensitive than ordinary collodion. 

 The gelatine process which I use is that patented by Mr. Kennett, 

 of Maddox Street. 



By means of this process good micro-photographs may be ob- 

 tained by the light of a paraffin lamp. When photographing at 

 night by paraffin light or by the magnesium lamp, it is not neces- 

 sary to use the long-focussed condenser ; the source of light being 

 stationary, it is more advantageous to use an ordinary achromatic 

 condenser, the mirror of the microscope being made use of as the 

 reflector. The exposure with paraffin light is necessarily long — 

 two minutes with low, and as much as fifteen with high powers. 

 Kepresentations of microscopic objects obtained by photography have 

 this advantage over drawings, that it cannot be said that they show 

 merely what the observer thinks he sees. Whatever is shown must 

 be there, but on the other hand one plane of focus only can be 

 represented, so that the sharp outlines of drawings must not be 

 expected, especially in the case of the rounded bodies of which the 

 majority of tissues are composed. — Bead before the Medical Micro- 

 scopical Society, November 19, 1875. 



