130 PHOTO-MICROGKAPHY 



Firth, of Cliftou Park Avenue, Belfast, and few that can equal him, save Mr. Gatrell, 

 of Barnes, whose work is of the most excellent quality ; * but Thum, of Leipzig, and 

 Moller, of Wedel Holstein, also supply slides of exceptional merit and perfection. We 

 think they can be obtained at Messrs. Watson & Sons, and at Baker's, both addresses 

 being in Hoi born. 



In photographing diatoms, apochromatics become almost a necessity — certainly 

 so with high powers — for the manner they seem to pick out the details devoid 

 of all secondary spectrum co]ours must be seen to be appreciated. Becently, however, 

 the new achromatics, when used with " screens," as hereafter explained, give very fair 

 results. 



Critical light is an absolute necessity, but the iiis will usually want a small amount 

 of closintr ; in other words, too large a " solid cone" is not advisable, but, if too small, 

 the minute markings will seem to have thick cumbersome black edges (see Fig, 2, 

 Plate TIL). No coloured glasses or screens are usually necessary with these objects. 

 Expose fully about five to twenty seconds and develop deeply, and clear afterw^ards if 

 necessary. Great care in centreing the condenser is obligatory, and the light must be 

 narrowly watc^iccl to see it is quite even. If a good negative be procured it can be 

 subsequently enlarged to almost any amount. We have experienced no difficulty in 

 producing a secondary enlargement up to 1800 diameters on Ilford rapid bromide 

 paper in the manner already described of the diatom shown Fig. ^, Plate III. 



If achromatics be employed instead of apochromatics, it is mostly necessary to 

 employ a screen. A yellow one is good, but personally we have found a pot-green glass 

 give better results with an Edwards' Isochromatic medium plate (Fig. 1, Plate III.), 



(ii) Taking the Photographs with Different Methods of Illuminating 

 the Object. — Taking a photograph by direct light is that which has just been 

 considered for it is the ordinary method (i) as shown in Figs, 24A and b. We now 

 proceed in this section to explain how to take the photograph with the other different 

 forms of illumination described in Section I. of this chapter. First by reflected light. 

 As before stated reflected light is only employed in photo-micrography when using 

 an upright apparatus shown in Figs, 25, 26, 27, pages 56, 57. It has been also stated 

 previously, that nearly always the flat side of the mirror is used for high-power 

 objectives and the curved for low-power ones such as the inch. This may not always 

 be the case : the prudent observer should try both. 



* Whilst these pages are passing through the press Mr. Gatrell has sent us some amphipleura pellucida mounted in 

 realgar and other diatoms in quinidine and piperine which are of the highest order of merit", especially the amphipleura 

 pellucida, which of late have been so difficult to obtain. 



