instrument by such a maker as the late Andrew Ross, and he 

 must be struck with the diflPerence in the perfection of its 

 optical and mechanical parts, and be read)' to say to himself, 

 what a pity that man was not more of an engineer, or had 

 not taken hints even from a good scaffolding carpenter. 

 Take, for example, the adjustment of an objective for 

 different thickness of covering glass. At present the front 

 combination is screwed forward or backward, so that as it 

 approaches the middle, it recedes from the object, and vice 

 versa, the result being that the object is thrown out of focus 

 and the adjustment made, comparatively speaking, at hup 

 hazard. Yeai's ago Lister pointed out that the adjustment 

 ought to be made by altering the position of the other portion 

 of the objective, leaving the distance between the front and 

 the object unaltered, by which means the observer could see 

 when the best effect was obtained. One or two object glasses 

 were made for him in this manner, but (it is almost a shame 

 to have to say so) the mechanical difficulty of manufacture 

 was so much greater, that though the objectives were very 

 superior, no more have been made. Can we believe that if a 

 real mechanic gave his attention to the subject this state of 

 things would long remain ? Again, we all know the 

 necessity, both in microscopes and photographic cameras, for 

 adjusting the aperture of a lens to the work it has to perform, 

 and we know how perfectly this is done in the eye, the pupil 

 expanding or contracting under different circumstances 

 exactly to the extent required. Compare this with the sets 

 of stops with different sized holes, which we are accustomed 

 to put into our lenses one after another, till we get the best 

 effect we can, but which we often feel is not just the thing 

 we want, and you will at once feel the want of a good 

 mechanically adjusting diaphragm. AVell, at last an 

 approach has been made to this by what has been called 

 the Iris diaphragm, one of which I have in this condenser. 

 It consists, as you see, of four thin pieces of metal, each with 

 a notch cut in the end and placed over the lens in such a 

 manner as to leave an octagonal aperture. Each of these 

 pieces has a pi'n on it, and over them is a circle of brass with 



