Hi, I'm not really good at physics, that's why I truly appreciate your help on this one. I'm doing a project about optics, and I've never studied optics at school before, and because my time is limited, I had to skim through the materials so I'm not really sure if what I do is right. I'm constructing an apparatus to measure sugar concentration in water at 30 C degree, and I have done an experiment to calibrate it. Btw, in the experiment I used an apparatus, which I don't know its English name, so I really appreciate it if someone can give me an idea. Here's the description: It's used to measure the refractive index of liquids. There's a circle with the diameters crossing each other, and a scale next to it. I had to adjust the scale with a knob so that the line between dark part and the light part seen in the circle goes through the intersection of the two diameters. Here's the construction I thought of: there are two pieces (as below). The first one is a transparent glass, with basically two slots for the second piece to cling in and slide up and down. The second piece (sketched below) has a layer of prism glass on top and normal glass as shown. There's a laser source attached to it, which will point a laser beam down at an unchanged, known angel when turned on. On the body, I will have numbers of sugar concentration (as in a contractor), calibrated according to my experiment and some computations. https://www.physicsforums.com/attachment.php?attachmentid=10868&stc=1&d=1189566894 Here's how it works : you put the solution inside the glass, slide the 2nd piece down so that the prism glass sit on the liquid (immobilized by some kind of stop, I don't really have an idea how yet). Turn on the laser beam and watch it being refracted as it goes through the prism layer... and read the number the refracted ray pointing at. I didn't have enough resources to actually build this apparatus to see if it worked properly, and also didn't have a strong enough theory background to be sure if everything was correct and would worked that way... so I'd really appreciate it if you check it for me theoretically, or give any suggestion to improve it. Thanks a lot!