468 Opaque Illuminators for High Powers. 



an arrangement substantially the same as theirs. Instead of 

 using a piece of glass with the two sides ground parallel, Mr. 

 Beck mounted a disc of thin covering glass, so that it could be 

 placed across the tube of his apparatus at an angle of 45°, or at 

 such other angle as might for special purposes be desired. Pie 

 did not introduce the diaphragm which Messrs. Powell and 

 Lealand copied from Mr. Smith's pattern. Messrs. Powell aud 

 Lealand devised their plan a few days before Mr. Beck devised 

 his, but the latter gentleman was the first to exhibit it in public 

 at the last meeting of the Microscopical Society, previous to which 

 Mr. Lobb had received their instrument from Messrs. Powell 

 and Lealand. This rapidity in devising what to both of them 

 was a novelty, is highly honourable to both firms ; but on refe- 

 rence to Mr. Smith's not very clear description, it seems pro- 

 bable that he had tried the very same plan and rejected it as 

 less desirable than his own. His words are, Cl I attempted at 

 first to employ as the reflector a disc of the thin glass, or two 

 or th ree of them, used as covers of microscopic objects.'''' 'The 

 words we have italicised may throw a doubt as to how he used 

 the thin glass, as we cannot imagine he would put one behind 

 the other in the same plane. He found this plan, however he 

 carried it out, gave a field of " peculiar flatness," that is to say 

 his objects were not in sufficient relief, and he states that on 

 this account he preferred the unilateral illumination from a small 

 silver mirror placed on one side, so as to occupy apposition some- 

 what near the letter B in Fig. 2. 



We have tried Messrs. Powell and Lealand' s and Messrs. 

 Smith and Beck's pattern, and both give highly important, and 

 approximately good results. Neither the thin glass, nor the 

 thicker glass with parallel sides, interfere as might be expected 

 with the definition of high powers; on some accounts we prefer 

 the thin glass ; but it is easily broken when it requires wiping-, 

 and very easily replaced. The power to move the glass in Mr. 

 Beck's plan we look upon as an advantage, and with regard to 

 Powell and Lealand's we find their diaphragm decidedly useful. 



Those who wish to study the effects of vertical illumi- 

 nation, as compared with oblique illumination, should ex- 

 periment with the admirable reflector made by Mr. Beck 

 for Mr. Sorby. When oblique light is wanted for powers of 

 one and a half or one inch, or two- thirds, this is by far the 

 finest illuminator we have tried, and in a moment a plane 

 mirror can be put in such a position as to substitute vertical 

 illumination for oblique. The effect is marvellous with many 

 objects. Penetration is instantly lost, and every minute surface 

 marking wonderfully shown. Thus special information of a 

 very valuable kind is gained. When vertical illumination is 

 applied to high powers, their penetration is much lessened, but 



