48 



THE AMERICAN MONTHLY 



[March, 



rior side, holding the back towards 

 you, was first ground flat and after- 

 wards slightly concave from back to 

 edge. A similar knife I find is fig- 

 ured in Mr. Rutherford's ' Outlines 

 of Practical Histology.' 



A list of soine of the vegetable ob- 

 jects I have found most interesting 

 may be acceptable to some of your 

 readers : — 



Leaves. — Drosera rotiiiidlfolia^ 

 Dionea muscipula^ Hepcitica tri- 

 loba^ Oxalis stricta^ Jlava^ hirsuta^ 

 and B owiei ; Deu tzia gra cilis , 

 cruenta and Fortunii ; T'radesca.n- 

 tia zebrina^ Eucalyptus globtilus^ 

 Btichu serratifolia^ Cassia acuti- 

 folia^ Rhus Toxicodendron, Adi- 

 an tu in cuneatum a n d pedatum , 

 Pier is serr^ilata^ Elaeagmis. 



Sections of Stems and Midribs. — 

 Ficiis elastica^ Strelitzia Regina, 

 Alt /leva rosea ^ Asclepias cornuta^ 

 Riibus villbsus^ Inipatiens Balsain- 

 inia^ Pteris aquilina ixndserrulata, 

 Paulownia i?nperialis. 



Sections of Stems. — Aspidium 

 Filix mas, R i c i n z( s co ni ni u u is , 

 Miisa sapient ium. Euphorbia spl en- 

 dens. Datura stranioniuvi, Dra- 

 ccena Braziliensis, Aila)ithus. 



Photo-Micrograpliy . — I V. 



BY THE EDITOR. 



{^Continued from page /o.] 



3. Illumination. 



It is not unlikely that some of our 

 readers have been surprised at a re- 

 mark made in the course of these ar- 

 ticles to the eftect that, in certain 

 cases, lamp-light may be even better 

 than sunlight. The statement, how- 

 ever, was not carelessly made. In 

 this article wx have to consider the 

 various methods of illumination that 

 are used, and it will be well first to 

 briefly notice the peculiarities of the 

 light from diflerent sources. First, 

 it should be observed that the lisfht 

 that acts most rapidly upon the sen- 

 sitive photographic plate is that 

 which is found in the blue portion of 

 the spectrum, the maximum action 



upon bromide plates being between 

 the Fraunhofer lines f and g, the ex- 

 act position varying with the natui'e 

 of the sensitive emulsion. The usual 

 range of sensitiveness of the ordinary 

 commercial plates is between lines f 

 and H, diminishing almost abruptly 

 below F and more gradually in the 

 violet and ultra-violet, extending as 

 far as n. In other words, the great- 

 est sensitiveness is in the blue and vi- 

 olet, not as some have supposed, in 

 the ultra-violet. For this reason, it 

 may be incidentally remarked, the 

 visual and actinic foci are coincident 

 if we focus with blue light. The 

 ordinary dry plates, however, are 

 acted upon by yellow light if the ex- 

 posure be long enough. For this 

 reason the plates are handled in ruby 

 light in the developing room. 



From this we can understand the 

 reason for the assertion that lamp- 

 light may possess some advantages 

 over sunlight in photographing par- 

 ticular objects. Take, for example, 

 a preparation having much yellow, 

 chitinous structure. Lamp-light 

 being deficient in blue rays, a long 

 exposure can be given with the yellow 

 rays passing through the object be- 

 fore the blue of the transparent field 

 has weakened the other portions of 

 the plate by over-exposure. In prac- 

 tice, however, this will only be 

 found advantageous in particular 

 cases, and we do not advise the use 

 of any artificial light when the sun- 

 light can be used. But probably by 

 far the greater number of those who 

 use the microscope are obliged to 

 work at night, and for them artificial 

 light of some kind is necessary. 



We cannot, in consideration of 

 what we have seen, fully agree with 

 the opinions expressed by our able 

 correspondent, Dr. Miller, on page 

 19. What we need in photography 

 is a light that will aflect the sensitive 

 plate, and having that we can take 

 good photographs. 



In the days of collodion, plates 

 were far less sensitive than the mod- 

 ern dry plates, and clear sunlight was 



