Is a thermoscope a reliable instrument for measuring temperature?

[Portuga]

Hello, gentleman! I was reading the chapter 18 of Halliday's textbook (9 ed.), and was terrified by an section about the zeroth law of thermodynamics. The auctors said in it that thermoscopes are not thermometers, because their measurements don't have physical meanings.
Later, they presented one example, in which a device like a thermoscope had a special feature: registered same measurements for objects which were in thermal equilibrium. Ok! Then they said something like "now this is a thermometer, and we just need to calibrate it".All right. My hypothesis are:

1. A thermoscope is only able to detect the variations of temperature. We can't trust in its measurements, I mean, we are not sure that bodies with same readings are at same temperatures.
2. A measurement only has physical meaning if we can trust in it.
3. A thermometer gives the same readings for objects which are in thermal equilibrium.
4. We need to calibrate the thermometers so we can be sure that two equal readings correspond to the same temperatures.

Am I wright? Did I miss something important?

 

[Infinitum]

Yep. You are right. The key idea is that thermoscopes are not calibrated, which means you never know what a particular reading signifies. It is used to measure only change in the temperature. Adding an appropriate scale to the thermoscope by comparative observations would convert it into a thermometer.

In your second point, I would just like to note that 'trust' should not necessarily be exact. Measurements are never exact, there is always a degree of uncertainity/error associated with every measurement, and the knowledge of this while using any instrument is highly crucial.